The man in the flat opposite the townhouse waited for Damascus to call Almussab. There may have been a leak, Almussab was told, he should move to the second address that had been rented for them and he should move his people as quickly as possible. When Almussab put the phone down, he gave the order and the five others set about packing their possessions and equipment and sanitizing the townhouse. Every surface in the kitchen was wiped clean and all food was removed, the bathroom was scoured and bleach poured down the waste pipes, basin and shower. The dining and living rooms were cleaned of fingerprints and all the carpets were vacuumed. The vacuum and all bedding was loaded into the people carrier, along with the contents of the kitchen waste bin.

He watched the garage door open and the vehicle drive through it, then the man in the flat waited for half an hour before he made his way across the road and let himself into the townhouse with a key he had been given. He looked around the property and nodded; they had done a thorough job. In the dining room, he pulled out a drawer in the sideboard. Taking a crumpled piece of paper from a briefcase he carried with him, he pushed it into the gap between the drawer and the back of the sideboard, then closed the drawer. With one final look round the townhouse, he left quietly, locking the door behind him.

He drove to a quiet area off Marietta Street and dialled a number on his mobile phone. He told the police that there was something suspicious about the townhouse, a lot of foreign-looking people coming and going, as if they were planning some terrorist plot. He didn’t know if they were still there or not, but the place looked quiet now. No, he didn’t want to give his name. He hung up. Wrapping the phone in a newspaper and putting the parcel into a plastic bag which he knotted, he drove along the road to a strip shopping-centre development. Driving through the car park, he dropped the bag into a waste bin without getting out of the car.

Denning watched Doug McGuire carefully as he came into the room; there was an urgency in his movements.

‘We may have got them,’ said McGuire.

‘Says who?’

‘We’ve had an anonymous phone call. He said they were holed up in a house in the Crosscreek development in the south-west of the city.’

Denning nodded. There was always an anonymous phone call. Sometimes they were useful, but mainly it was a misguided lunatic or deliberate misinformation. But they all had to be treated seriously.

‘What’s the address?’ He asked.

‘15635 Bayshore Drive.’

‘Get me an aerial photograph of Crosscreek,’ said Denning, walking to the desk where Levi Cohen was sitting at an open laptop computer.

Cohen punched several keys on the computer and a photograph of the development came onto the screen, marked with the names of the major roads. Denning leant forward, frowning at the screen.

‘Doug,’ he said, ‘can you order units to block off the ends of the Bayshore Drive and, just in case, the entrance to Crosscreek. No-one goes in or out unless I say so. If anyone wants to know what it’s about, tell them we have a hostage situation.’ He turned back to Levi Cohen.

‘Can you get me some shots of the townhouse?’

Cohen went through a search engine for the names of the major estate agents, then scrolled rapidly through their websites.

‘This isn’t it,’ he said as the picture of a two-storey townhouse filled the screen, ‘but it’s the adjoining property and it’s identical. Here’s a shot of the back of the place.’

The photograph on the screen dissolved, changing into another, showing sliding glass doors to a patio and, on the first floor, a balcony.

‘I’ll get these printed off and see if the City Planning Department have detailed plans of the interior.’

‘Drew,’ said Denning, laying the two photographs on the desk in front of him, ‘I need your guys in three pairs, two through the front door, two through the garage and two dropping onto the rear balcony from the roof. Time is now the first enemy, get your equipment together and we’ll fine tune any plans in the van on the way over there.’

In the police van, the six men sat in the pairs O’Connell had ordered. He had teamed himself with Al Gomez, Cohen with Manchetti and Macleod with Schwartz. He and Gomez would go through the front door, Cohen and Manchetti would take the garage, leaving Schwartz and Macleod with the tricky drop onto the balcony. When Denning had finished talking to them, they sat in the van in the quiet mood experienced soldiers go into just before battle.

Most of them carried MP5s, compact and versatile army machine guns. Schwartz favoured a HK416, the Heckler & Koch carbine, and Gomez had a Mossberg pump action shotgun across his knees. All of them carried Sig Sauer pistols in holsters sewn into their black combat jackets. Gomez, Manchetti, and Macleod had pouches slung over their shoulders containing stun grenades, smoke bombs and tear gas.

Either side of the main gates of the residential community were two sweeping walls, painted sand colour. On each wall, picked out in two foot high dark blue tiles, was the word ‘Crosscreek’. Between the walls, at the end of the wide central strip of grass that divided the one-way street in and out, was a fountain, painted blue to match the tiles. The entrance was now blocked by two police armoured-vans, in front of which were a dozen police officers in riot helmets and body armour. The driver stopped the van and Denning, Curtis Hoffman and McGuire got out. The senior officer snapped a sharp salute to his Deputy Chief but looked at the other two with suspicion. High above them, Denning could hear the sound of a helicopter flying a pattern of tight circles.

He rested his elbows on the bonnet of one of the armoured vans and looked at the street through a pair of binoculars.

‘Which one is it, Levi?’ he called over his shoulder.

‘Second on the left, just past the side road, as the street bends to the right.’

Denning took his time observing the house. He stood motionless, leaning against the van for over a minute. There was no sign of life. That could mean the house was empty, or whoever was in it was just waiting patiently for them.

He pushed himself away from the van and walked back to the vehicle where the six men were sitting.

‘Get yourselves ready, gentlemen.’

With no signs of anxiety or haste, the men put on throat microphones and earpieces, strapped on helmets and hung gas masks round their necks. They checked their weapons and unloaded the equipment they would need from the back of the van. Levi Cohen set up a laptop computer on the driver’s seat and Denning sat in the front passenger seat and looked at the multiple images on the screen. Each man had a camera attached to the side of his helmet and there was a view of the front of the house from the camera set up by the snipers on the rooftop opposite. He put on a pair of earphones and adjusted the microphone attached to them.

At Denning’s nod, one of the armoured trucks reversed six feet and Schwartz and Macleod stepped through the gap. They ran to their left until they made the safety of a hedge that marked the line of a path running beside a small stream. The hedge hid them from the house and they jogged along the path until they were level with the house just beyond the target townhouse.

The townhouse they were to attack was the middle of a terrace of three. Crouching, they ran to the gable end of the house at the far side. There was a ledge over the internal staircase and, once on the ledge, they could pull themselves up onto the roof. Schwartz opened a telescopic ladder, a central pole with alternate small metal steps either side of it, and leaned it against the wall. They both climbed it and hoisted themselves onto the roof. They made their way silently across the rows of barrel tiles until they were over the upstairs balcony of the middle house. Using the guttering for leverage, they dropped onto each side of the concrete balcony surround. Macleod looked carefully round the corner of the wall and through the sliding glass doors into the bedroom. The room was empty. He pulled a sledgehammer, its shaft shortened by sawing it in half, from a harness on his back and waited.

While Jerry Schwartz was erecting the ladder, Levi Cohen and Charlie Manchetti ran to the large double up and over garage door. Manchetti had been pulling a cart with two small cylinders on it. Kneeling down by the doors, he ignited the brass nozzle at the end of the pipes that ran into the cylinders and adjusted the flame of an oxyacetylene burner. He pulled on a pair of dark goggles and put the flame to the metal door. In ninety seconds, he had cut a hole in the door three feet square. Cohen gently pulled the rectangle of metal away while Manchetti shone a torch into the dark interior of the garage. There was no sign of people, motion sensors or tripwires. He nodded to Cohen and they both crawled through the hole and made their way across the garage floor to three steps that led up to the kitchen door. Manchetti gently tested the handle; it was locked, but it was an internal door and the lock didn’t look high security. He pulled a crowbar from his belt, measured against the door and stood, waiting for the order.

Andrew O’Connell and Al Gomez were only twenty yards behind Cohen and Manchetti. They passed them as Manchetti was cutting the garage door and walked quickly up the short path to the front door. O’Connell examined the front door for thirty seconds. It was made of good, solid wood with two leaded-glass panels running from the top to the height of the door handle. Blinds had been lowered behind the glass, so he could not see in and thankfully, anyone inside could not see out. He studied the lock; too sturdy to force and too good to pick. He pushed a palm full of plastic explosive into the lock and the thin gap between the door and the jamb. Pushing a radio-controlled detonator into the explosive, he stood back with a small transmitter in his hand.

As he stood by the door, O’Connell felt a trickle of sweat run down the small of his back. He closed his eyes and tried to control the shaking that was getting worse by the second, the churning in his stomach and the beads of sweat on his forehead and upper lip. He could feel Gomez looking at him and had the sudden urge to give the transmitter to him and walk away. He leaned back against the wall, praying he wouldn’t have to go through the door. He knew he would collapse if they had to wait for long.

All six men waited, Denning could hear the short blasts of nervous breathing through his headphones and see the doors and windows of the house that their helmet cameras were showing.

‘Ten seconds,’ he said calmly.

‘Five.’

‘Go.’

In the same moment Macleod hit the doors with the sledgehammer, the glass crazed and fell onto the balcony floor in a shower of diamonds, Manchetti crowbarred the kitchen door and O’Connell pressed the button on the transmitter, the lock exploding in a storm of twisted metal and splintering wood.

As O’Connell and Gomez ran into a short hallway, they could hear Manchetti and Cohen in the kitchen to their left. All six had studied and discussed the plans and knew where they were going. O’Connell and Gomez took the living room, study and ensuite bedroom downstairs. Cohen and Manchetti ran up the stairs to join Macleod and Schwartz, who had come through the first floor window and were starting to search the other bedrooms, bathroom and upstairs den.

Denning watched the multiple images on the screen and heard the word ‘clear’ as each room was searched. Then there was silence as each man stopped, knowing that there would be no firefight and let his adrenaline levels start dropping back to normal.

‘All clear, Doc,’ said Andrew O’Connell eventually.

Denning leant back in his seat, his hands clasped behind his head.

‘What do you make of that?’ asked Curtis Hoffman, who had been watching the computer screen through the open passenger window.

‘They knew we were coming,’ said Denning very simply, ‘and that is a major worry.’

The six men had gathered in the living room. They were relaxed, shouldering their firearms and packing the equipment when there was the sound of the front door being quietly opened. Four submachine guns, one carbine and a shotgun levelled at the hallway as the men spread out and took cover behind pieces of furniture. There was a strange rustling sound and the soft padding of pairs of feet in soft soled shoes.

Two men in white plastic suits and carrying aluminium briefcases came unhurriedly from the hallway into the living room. They saw the guns aimed at them without any sign of surprise.

‘Relax boys,’ said one, ‘we’re the Cavalry, not the Indians.’

‘Who the hell are you?’ demanded Sergeant Schwartz, his carbine still aimed at them.

The man who had spoken reached inside his shirt. He moved slowly as the sergeant seemed nervous, and took out a small gold shield.

‘FBI Terrorist Squad Search Team,’ he said, holding the badge up to the sergeant’s face.

The search took over five hours. The two men worked methodically through each room, dusting for fingerprints, taking tapings from surfaces and using mirrors and torches to reach into the darkest, unseen corners and crevices. They knew early in the search that they were dealing with professionals who wouldn’t leave any traces unless they were careless. The search was nearly three hours old when one of the men, pulling out sideboard drawers, found the crumpled piece of paper, wedged between the back of the drawer and the frame of the sideboard. By the time the search ended, it was the only thing they had found. They bagged it and took it to the local FBI offices for forensics examination.

Sitting well back from the window in the darkness of the room, the man in the apartment opposite the townhouse had patiently watched the six police officers come out of the building and load their equipment into the back of their van. He had seen the FBI men change into white suits outside the front door and now watched them leave. One of them was holding a plastic bag with a crumpled piece of paper in it. He smiled to himself and waited for the men to get into their car and drive away. The apartment had been cleaned and all his possessions were in a suitcase in the Town Car downstairs. He had arranged to deliver the keys of the apartment to the estate agents and had rented another one down the street from the house to which Almussab and the others had just moved.

He got up, locked the door, went downstairs to the garage and drove the Town Car out of the development.

The call from Damascus hadn’t surprised Abu Almussab; he knew that the controllers liked to keep a watchful eye on the foot soldiers in any operation. When they told him to move house they also told him about the packet in the left luggage at the MARTA station and where the key could be collected.

If Faqir Rashid thought he would have time to settle into his new accommodation, he was wrong. He had just thrown his kit bag onto the nearest bed when Almussab told him to go to Five Points MARTA station, where he could find the locker key. Rashid nodded and went to the garage to start the van. As he watched him drive off, Almussab thought that they had had the van long enough, it was time to get rid of it and rent another, from a different company.

Within the hour Rashid was back. He silently handed the envelope to Abu Almussab, who took it to the dining table, opened it and spread the papers across the polished surface. He sat reading, his face expressionless, for ten minutes.

‘Hussein and Hanjour, come here,’ he said quietly.

The pair moved to the table and stood over him.

‘I want you to make another bomb,’ he said to Rahan Hussein, ‘I want it to be small, but powerful.’

‘How small and how powerful?’

Almussab shrugged.

‘The size will depend on what we put it into. At a guess, no larger than 40 centimeters square. The power? Enough to destroy a room and everyone in it.’

Hussein took a notebook from the cargo pocket in her trousers and started writing in it.

‘How large is the room and what furniture is in it?’

‘I will tell you tonight.’

‘How many people?’

‘Does it make a difference?’

‘A couple, no problem. More than that, if you want to make sure you kill them all, I’ll have to pack the explosive with ballbearings and scrap metal.’

‘Do it.’ Almussab turned to Hanjour, ‘These papers tell me who the people are who were sent to find us. The one with the brains, the one who will be directing them is a man called Marcus Denning. Without him they are a bunch of headless chickens. I want you to kill him.’

‘Do you have a photograph of this Denning?’

‘No, he’s a very careful man. I have never seen a photograph of him. I saw him face to face some years ago and he can’t have changed that much. I need you to take some photographs of the people in the office and I will identify him to you. Here is the address of the office they occupy. If you leave now, I will arrange for the blinds of the office windows to be opened some time this evening.’

The brooding, impassive expression remained on Hanjour’s face. He got up from the table and went to the bedroom where he kept his luggage. Sometimes, Almussab wondered what went on behind those dark eyes.

Hanjour unzipped one of the suitcases and took out a small hand grip. He opened it to check the contents. Inside was a Nikon D700 digital camera with a Sigma 500 mm telephoto lens. Closing the bag, he left the house without a word. He would walk to Garnett station and take the train to the city centre, losing himself in the early evening crowds.

‘You know what to do, get started,’ said Almussab to Hussein.

As Rahan Hussein got up from the table, Almussab motioned for Ibrahim Khalil to join him.

‘It’s a very warm afternoon,’ said Almussab softly, ‘there is a suite of offices on the second floor of the police headquarters, it’s suite 208. I want you to disable the air-conditioning supply to it for three hours.’

‘When do you want this done?’

‘Now. And I don’t want you to leave any evidence of what you’ve done.’

‘It can’t be a straight bypass system, I’ll have to override the computer and scrub the program.’

‘Then you’d better get on with it.’

Five minutes later, Khalil set off on foot with a laptop case slung over his shoulder. He was wearing a golfing visor, polo shirt and long cargo shorts. He looked like any one of the thousands of students who inhabit Atlanta.

The bus service was good and Khalil arrived at the back of police headquarters five minutes after Hanjour settled himself on the roof of a four-storey building opposite the suite of offices. He took out the camera, checked it and knelt down to wait. Hanjour had had lots of practice at waiting.

The keypad on the back door was no match for the program in Khalil’s computer. He attached a sensor to the front of the lock and ten seconds later, he tapped the five digits that unlocked the door and was into the building. The air-conditioning system was governed by a unit in the basement. He tested the metal doors of the cabinet that held it, and they were unlocked. At one side of the loops of wiring was an electronics board. This ran the system through a remote computer somewhere in the workshop of the company who subcontracted the heating and cooling system. Khalil ran his fingers down the side of the board and found the USB port the engineers would use to check the heating and cooling. He plugged his laptop into the port.

Whoever had set up the system had not seen the need to protect it with a password. He tapped half a dozen keys and within seconds the entire system was displayed on his computer screen. He isolated Suite 208 and inserted a program to turn off the air conditioning for three hours, long enough for the suite to get uncomfortably hot, but not enough for a service engineer to be called out. After three hours, the program would switch the air conditioning back on, then rewrite itself as if it had never existed. Only a forensic computer expert could break through the barriers and find the program and, now that the system would be working again, no-one would bother to do that.

Two hours later, as the temperature in the suite of rooms rose towards 85°, Levi Cohen moved to the windows, drew back the blinds and opened the four transom windows. On the rooftop opposite, Ziad Hanjour rested the camera on the parapet of the roof and pressed the shutter button, moving the camera slowly across the entire suite. The electronic drive took five shots a second for twenty seconds, until the extra-long, light intensifying film he had loaded into the camera, ran out.

Denning was sitting with his back to the widow, reading intercepts of the telephone traffic crossing the Middle East, when he felt the cooler air on the back of his neck. He turned.

‘Shut those damned blinds!’ he shouted, getting up so suddenly he sent his chair crashing to the ground.

Cohen pulled the blinds shut.

‘The A.C.’s gone, we needed some fresh air, it is getting like a sauna in here.’

‘How long have you been in the Atlanta force?’ asked Denning, carefully pushing the corner of a blind an inch to one side and looking out at the streets and buildings opposite.

‘About six years.’

‘Ever known the air conditioning in this building to fail?’

Cohen shook his head.

‘Everything happens for a reason,’ said Denning, letting the blind slide back into place.

Ziad Hanjour loaded the camera back into its case and turned away from the edge of the building. He made his way in a low crouch until he was out of the eyeline of the suite of offices, then rose to his feet and walked slowly to the head of the stairway that led from the roof to the pavement, three floors below. He retraced his steps, taking the MARTA to Garnett and keeping his cap low over his eyes, pretending to sleep for most of the journey. By the time he had walked the half mile from the station to the house, Khalil had set up a darkroom in one of the smaller bedrooms and was waiting for him.

Half an hour later, the photographs had been reduced to twenty clear shots and were spread out on the dining table. The terrorists gathered round them.

‘That,’ said Abu Almussab as he stabbed one of the photographs with his finger, ‘is Denning.’

Hanjour turned the photograph towards him and looked at the man with grey hair and glasses who was moving towards the window, shouting at someone. He did not look too formidable, but Ziad Hanjour never made the mistake of underestimating his enemy.