`Exactly,' Graves said.

`Wherever he went, he wouldn't be safe. He would be a mass murderer and he would have left a very clear trail behind him.' He gestured at all the equipment. `Would he really leave such a clear trail for others to follow?'

`It must be true,' Nordmann said, getting excited. `He had to have two purposes - first to discharge the gas, and second to obliterate the evidence.'

`Obliterate the evidence how?' Phelps asked.

Graves leaned on a tank. He turned to Nordmann. `How long would it take this cylinder of gas to discharge?'

Nordmann shrugged. `Ten or fifteen minutes.' Then he said, `I see. You want to know exactly.'

`Yes,' Graves said.

`Why exactly?' Phelps asked.

Graves ignored him.

Nordmann said, `Normal Army pressure tanks are usually stabilized at five hundred pounds per square inch. So these tanks… Anybody got a tape measure?' He looked around the room. One of the cops had a tape. Nordmann measured the tank. `Thirty-seven inches in circumference,' he said. `Eight feet long, that's ninety-six inches, with a radius of…' He wrote on a small pad, doing rapid calculations.

Phelps said to Graves, `Why do you need it exactly?'

`Because,' Graves said, 'Wright didn't care if we broke into this room. He had another contingency plan to cover that. And we need to know when it will take effect.'

Phelps looked totally confused.

`A radius of six inches,' Nordmann was saying. `And a length of ninety-six inches gives a volume… well, figure for a cylinder… at five hundred psi… let me check the nozzles…'

He wandered off. Graves said to a cop, `What time is it?'

`Five oh seven, Mr Graves.'

Nordmann finished his calculations and turned to Graves. `At normal discharge rates, it would take these tanks sixteen minutes to empty.'

`That's it, then,' Graves said. `At five sixteen, a bomb will go off in this room, destroying everything. We've got to find it.'

Everyone paused. They stared at him.

Phelps said, `A bomb?'

`Of course. That's why he knocked out the elevators - to trap us here. In case we managed to disengage the mechanism, he wanted us here when the bomb went off, releasing the gas and eliminating the evidence.'

Phelps said, `But there's no evidence of a bomb '

`Remember the sniffer?' Nordmann asked.

Phelps frowned.

`The sniffer,' Graves said, `picked up oxides of nitrogen. Plastic explosive.'

`Yes…,

'Okay,' Graves said. `Where is that explosive now?'

Phelps looked around the room. `I don't see it anywhere,' he said.

`But the sniffer detected it.'

`Yes…'

`There must be a bomb,' Graves said. `And it must be in plain sight.'

`Five oh eight,' a cop said.

`We better get these tanks out,' Nordmann said. `We don't want them damaged by the bomb.'

`Right,' Graves said. `And let's get the sniffer in here. It'll help us find it.'

The sniffer had been taken to the other building. Phelps turned on the walkie-talkie and talked to Lewis. Lewis said he would bring it as soon as he could, but it would take time to climb nineteen flights. Phelps told him to hurry and added a string of expletives.

Meanwhile Nordmann supervised the removal of the tanks to the hallway. The cops carried them, four men to a cylinder, grunting under the weight. Graves searched the room - scanning the wall surfaces, the door, the window ledges for any irregularity, any discontinuity that would suggest the location of explosive. Plastic explosive, Compound C, could be shaped and moulded into a variety of forms. That was its advantage.

It could be anywhere.

Nordmann stuck his head into the room. `Maybe you should get out of here,' he said. `We can let the room blow if the tanks are far enough away. No sense in risking anything.'

`I'll stay until we find it,' Graves said. He walked to the window and looked out. He saw Lewis running across the street with the sniffer on his shoulder.

`Five ten,' Phelps said.

It would take Lewis at least two minutes to scramble up all those stairs. Graves stared out the window, wondering what was happening to Wright. Had they managed to cut his body out of the wrecked car yet?

Odd, he thought, how the game continues.

'Lewis is coming,' Phelps said.

`I saw.'

`How much explosive is supposed to be in this room?'

`Twenty pounds.'

`Christ.'

Graves continued to stare out the window. Where would Wright hide twenty pounds of explosive? What would be the supremely logical hiding place? Nothing less would satisfy Wright, he was sure of that.

He shifted his position at the window, careful to avoid the jagged splinters of glass around the sill. As he did so, he looked down at his shirt. There was printing on the shirt; some of the lettering from the tanks had come off on his arms and chest when he had leaned on them. '

" CHEMI," it said, and then faintly, " DO NOT."

He looked at his watch. 5:12.

`Where the hell is Lewis?'

Lewis appeared, running down the corridor redfaced and out of breath. `Sorry,' he said. `Came as fast as I could.' He turned on his sniffer and walked around the room, pointing the gunlike wand, staring at the dial on the shoulder unit.

Graves and Nordmann watched him.

Lewis began with the door, then turned to the walls. He checked carefully from baseboard to ceiling. In a slow, methodical way he went entirely around the room. Finally he stopped.

`You get a reading?'

`No,' Lewis said, checking the machine. `Nothing.' `Maybe it's in another room,' Nordmann said.

`I doubt it,' Graves said.

`Let's check it,' Lewis said. He disappeared into the bathroom, worked through it and through the adjoining bedroom of the apartment. He came back a moment later. `Maybe the machine's broken.'

`How can we test it?'

`Give it a smell of some kind.'

`Like what?'

`Anything strong. Cologne, perfume, food…'

Graves went to the refrigerator, but it was empty. When he came back he saw Nordmann strike a match,

Graves bent over the tank and rubbed the lettering. It streaked on his finger.

`Go to the window,' Graves said to a cop. `Use the bullhorn and clear the street below. Do you understand? Get everybody the hell away from the street.'

The cop looked confused.

`Go!'

The cop ran.

Graves pushed at the surface of the tank in front of him. His finger left a minute indentation. `That's where your plastic explosive is,' he said. `It's wrapped around the tanks in thin strips - strips pressed through the rollers of an old washing machine. There must be a timer…'

He ran his fingers quickly along the surface, feeling for lumps and irregularities. He couldn't find it, but he was in a hurry.

`Christ,' Lewis said, pointing his sniffer. `This is it. Plastic explosive.'

`The timer, the timer…'

`It's after five fifteen,' Nordmann said.

`Get those cops in the stairway away from that tank,' Graves said. `Tell them to drop it and run.'

His fingers raced along the surface, back and forth. But it was eight feet of tank - too much surface to cover easily. It was probably a small timer, too. Perhaps miniaturized, perhaps the size of a thumbnail.

`Damn!'

`I get it,' Lewis said. `That was why he wanted inflammable plastic for the tanks. It'll explode and burn without leaving a -'

`Coming up on five sixteen,' Nordmann said, looking at his watch.

Where was the timer?

`I can't find it,' Graves said. `Come on.' He picked up the tank by the nozzle and began dragging it back into the apartment. `Help me,' he said.

There were three of them, but the 500-pound tank was bulky. As they entered the apartment, the cop at the window was on the bullhorn saying, `Clear the area, clear the area.'