He used a bit of gum to stick the piece of veneer containing digital details of NSA’s surveillance program behind one of his front teeth. Then he went to the first motel room he’d entered, where another change of clothes hung in the closet. He showered, changed and left, walking along the street for a few blocks, then grabbed a bus, rode it to a rental car shop, slid into another leased ride and drove home.

He spent an hour digging the information out of the tiny device and another hour putting it in proper form for passing on. As a spy Seagraves had long been an enthusiastic student of secret codes and the history of cryptology in general. Nowadays computers encrypted and decrypted messages automatically. The most secure systems used keys consisting of hundreds or even thousands of digits — far longer than the actual messages being encrypted. At the very least, breaking the strongest of these keys required enormous computing power and thousands if not millions of years. This was so because modern–day cryptologists assumed that the coded messages would be intercepted and thus had engineered their encryption systems for that eventuality. Their mantra could be: You can intercept it, but you almost certainly can’t read it.

Seagraves had opted for a more vintage method of encryption, one that, because of the way the messages were communicated, might be even more unbreakable than the modern–day, computer–generated juggernauts for one simple reason: If you couldn’t intercept the message, you had zero chance of reading it. There was something to be said, he mused, for the old ways. Even the NSA, with all its technological might, could learn a lesson from that.

After he had finished that task, he fell into bed.

Instead of sleeping, though, all he could think about was his next kill. That would enhance his precious “collection” by one.

• • •

Back at his cottage, Stone quickly brought the others up to date on what they’d found. When he’d mentioned the hidden lettering on the cylinder reading “CO₂, 5,000 ppm,” Milton had immediately gotten on his laptop where he’d stored pertinent downloaded files from the Internet. After Stone had finished speaking, Milton said, “CO₂ is almost never used in occupied spaces because it can suffocate people as it instantly takes oxygen content out of the air to extinguish fires. At five thousand parts per million it would be rapidly fatal for someone standing nearby; he’d be overcome before he could escape. And it’s not a pleasant way to die.”

Annabelle made a coughing noise, stood and went over to look out the window.

“And I presume it has a cooling effect,” Stone said hastily, eyeing her with concern.

Milton nodded as he scanned his screen. “With high–pressure systems there’s a discharge of dry ice particles. They call it a snow effect because it rapidly absorbs heat, reduces ambient temp and helps prevent flash and reignition of the fire. The snow turns to vapor under normal temperatures and leaves no residue.”

Stone added, “By the time Caleb and DeHaven were found in the vault, the O₂ levels had probably returned to almost normal, and any lingering chill would be put down to the extraordinary levels of cooling in the vaults.”

“But if DeHaven were killed by CO₂ suffocation, wouldn’t that have turned up in the autopsy?” Reuben asked.

While they’d been talking, Milton’s hands had been flying over his keyboard. “Not necessarily. This is information I downloaded earlier from a site sponsored by a national medical examiners’ organization. While carbon monoxide poisoning can be detected postmortem by the cherry–red appearance of the skin, carbon dioxide exposure doesn’t leave such clear–cut signs.” Reading from the screen, Milton said, “The only way to detect low levels of oxygen in a person is through a blood gas test which measures the ratio of oxygen to carbon dioxide in a person’s blood. But that test is only done on the living to see if oxygen levels need to be increased. It’s never done postmortem for the simple fact that the person’s dead.”

Caleb added, “From what I was told afterward Jonathan was pronounced dead in the vault. He wasn’t even taken to the emergency room.”

Stone said, “The cylinder they removed with the label FM–200 was the one I focused on, for obvious reasons.”

“I’m not getting what you mean,” Reuben replied.

“The library’s scrapping the halon system. If I’m right and they brought in a cylinder full of deadly CO₂ with the wrong label to disguise it, they wouldn’t have been bringing halon back to the library; that would have raised suspicion.”

“Right, they’d have to bring in the gas they were replacing the halon with. FM–200,” Caleb added. “And they took it out tonight with a bunch of halon cylinders. If we hadn’t been there, no one would’ve noticed.”

Stone nodded. “And I’m certain that the cylinder connected to the piping tonight was full of halon. The empty cylinder that had contained the CO₂ was probably disconnected from the piping right after it was discharged. Then if the police happened to check, they’d find nothing out of the ordinary. They wouldn’t check every cylinder in the place, certainly. And even if they did check, they’d have to send it to Fire Control, Inc., for that purpose. I doubt they’d get an accurate answer back because whoever orchestrated this is obviously employed by the company.”

“The perfect murder,” Annabelle said grimly as she sat back down. “The question is why. Why would anyone want to kill Jonathan that badly?”

“That takes us back to Cornelius Behan,” Stone said. “Now we know that the lethal CO₂ cylinder that killed DeHaven was switched for the halon. We also know that Fire Control is owned by Behan. The man obviously had DeHaven killed. Behan showed up at the reading room to see Caleb on the very same day the cylinders were removed from the library. I’m sure he was trying to determine if there was any interest in the nozzle And there must be some connection between Behan and Bob Bradley.”

Reuben ventured, “Maybe Bradley and Behan were part of the spy ring we think is operating here. Bradley comes to visit Behan at his home, and Jonathan saw or heard something he wasn’t supposed to. Or he might have seen something that tied Behan to Bradley’s murder. Behan found out about it and had him killed before DeHaven could tell anybody and lead the investigation to him.”

Stone said, “It’s possible. We have a lot of ground to cover, so we need to split up. Caleb, you go into the vault first thing tomorrow and check behind that air–conditioning grille for evidence of a camera having been placed there. Next examine the video surveillance tapes for people going in the vault.”

“What?” Caleb exclaimed. “Why?”

“You yourself said that whoever killed Jonathan would have to have access to both the library and the vault. I want to know who went in that vault a few days before DeHaven’s death and then after he was murdered.”

“I can’t just walk into security and demand to see the tapes. What possible reason would I give?”

“I’ll help you think of one, Caleb,” Annabelle said.

“Oh, great,” Reuben said under his breath. “First Milton gets to play with the lady and now Caleb. But moi? Nooo.”

Stone continued, “Reuben, I want you to make an anonymous call to the D.C. police and tip them off about the CO₂ cylinder. Use a pay phone so they can’t trace the call. I don’t know if they’ll take it seriously or not. And by the time they get there, it’ll probably be too late, but we have to try.”

Caleb said, “But won’t that let certain people know that we’re on to them?”

“Maybe it will,” Stone said. “But right now that’s the only evidence we have that DeHaven was murdered. After you do that, Reuben, I want you to take up surveillance on Good Fellow Street starting tonight.”