The whole crew of them ended up on their knees, watching over the colonel's shoulder. He pulled off the outer grille to reveal the sealed rectangle of fiber designed to catch dust, dirt, hairs, mold, spores, smoke particles, and the like; the tiny prokaryotes themselves, if freed from their sporelike cases, would have slipped right through this barrier and gone on, possibly, even to penetrate the electrolytic resin barrier behind it, only to be destroyed at the last when they reached the central flash-unit.

At Miles's nod the colonel gave way to Dr. Weddell, who sat cross-legged on the floor and earnestly saturated the air around the vent with his atomizer.

"So what's he doing?" the colonel whispered to Miles.

Miles suppressed the reply, Were spraying for traitors. Pesky vermin this time of year, don't you find? "Watch and see."

Weddell then pulled an ultraviolet hand-light from his box, and directed it at the filter. A pale red fluorescence slowly grew more brilliant as the black light played over the surface.

"There you are, my Lord Auditor," Weddell said. "The vector encapsulations were caught in the filter, all right."

"Just so." Miles scrambled to his feet. "That's our baseline, then. On to the next. You there"—he pointed to the forensics tech—"document, bag, label, and seal all that, and follow as quickly as you can."

The parade took their positions and followed him once more. This time he led them to the Department of Komarran Affairs, where Miles asked the disturbed General Allegre to join the procession. They all fetched up crowded into Captain Galeni's cubicle-sized office, fourth door down the Komarran analysts' corridor.

"Do you remember ever personally visiting Galeni in here in the last three months?" Miles asked Illyan.

"I'm sure I stopped in a few times. I came down here almost every week, to discuss items in his reports of particular interest."

As soon as the forensics tech arrived, out of breath, the colonel from Janitorial repeated his performance with the cubicle's return-air vent, identical to the one in Weapons Room IV. Weddell sprayed again, liberally. This time, Miles held his breath. The results of this test could force a major fork in his planned strategy. If Haroche had anticipated him—there had been two missing capsules, after all.

Weddell, on one hand and his knees, played his black light over the filter. "Huh." Miles's heart seemed to stop. "There's nothing here that I can see. Do you see anything?"

Miles inhaled, gratefully, as the other men bent to examine the filter also. It remained a slightly dirty and now-damp white.

"Can you ascertain that this hasn't been changed since the last scheduled maintenance at Midsummer?" Miles asked the colonel.

The colonel shrugged. "The filters are not individually numbered, my lord. They're interchangeable, of course." He checked the report panel he carried. "No one in my department has done so, anyway. It's not due to be changed again till next month at Winterfair. It looks to have about the normal amount of accumulation for this point in its cycle."

"Thank you, Colonel. I appreciate your precision." He rose, and glanced at Illyan, who was watching stony-faced. "Your old office is next, Simon. Would you care to lead the way?"

Illyan shook his head, politely declining. "There isn't much joy for me in this, Miles. Either way your results come out, I lose a trusted subordinate."

"But wouldn't you rather lose the one who's actually guilty?"

"Yes." Illyan's snort was not wholly ironic. "Carry on, my Lord Auditor."

They trooped up three floors and down one to the level of Illyan s old office. If Miles had managed to surprise Haroche with his arrival in force, the general showed no sign of it. But was there maybe just a little discomfort in his eyes, as Haroche greeted his old boss and offered Illyan a chair?

"No, thanks, Lucas," said Illyan coolly. "I don't think we'll be here very long."

"What are you doing?" Haroche asked, as the colonel, practiced, went straight for the grille low on the wall to the right of his comconsole desk. The increasingly burdened forensics tech followed him.

"Air filters," said Miles. "You didn't think of the air filters. You've never been on space duty, have you, Lucas?"

"No, unfortunately."

"Believe me, it makes you very conscious of things like air circulation systems."

Haroche s brows rose as Weddell began vigorously spraying around the vent. He rocked back in his station chair, as-if-casually. He sucked thoughtfully on his lower lip, and did not ask, Have you considered my offer, Miles? He was a cool hand, and patient, and perfectly capable of waiting for the answer to emerge. No reason for him to flinch yet; whether the filters here were jammed full of vector encapsulations or not, it would prove nothing. Lots of people went in and out of Illyan s office.

"No," said Weddell after a moment. "Take a look for yourselves, gentlemen." He passed the black light along to Ivan and General Allegre.

"You'd think it would be here," commented Allegre, peering over Ivan's shoulder.

Miles had given it about a twenty-five-percent chance, personally, though he'd upped the odds after finding Galeni's vent clear. That left one of the conference chambers, or …

"Find anything?" asked Haroche.

Miles made a small show of going over and borrowing the hand-light from Ivan. "Not in here, dammit. I was hoping it would be simple. If the prokaryote vectors are caught in the filters, they show up bright red, y'see. We tested one, downstairs."

"What are you going to do next?"

"There's nothing for it but to start at the top of the HQ building and check every filtered air vent till I get to the basement. Tedious, but I'll get there in the end. You know I said if I knew why, I'd know who. I've changed my mind. I now think if I know where, I'll know who."

"Oh, really. Have you tried Captain Galeni's office?"

"First place we looked. It's clear."

"Hm. Perhaps . . . one of the briefing rooms?"

"I'd give odds." Bite, Haroche. Bite my hook. Come on, come on. …

"Very good."

"If you want to save steps," put in Ivan, on cue, "you ought to start with the places Illyan went most, and work out from there. Rather than from the top down."

"Good thinking," said Miles. "Shall we start with the outer office? Then—excuse me, General Allegre, but I must be complete—the offices of the department heads. Then the briefing rooms, then all the affairs analysts' offices. We should probably have done the whole of Komarran Affairs while we were first down there. After that we'll see."

From the look on the forensics techs face, he was mentally kissing his dinner good-bye, a regret perhaps blunted by his obvious fascination with the proceedings. Allegre nodded; they all straggled back out, and the colonel began the drill again with the grille in the outer office. Miles wondered if anyone had noticed yet that Weddell didn't have nearly enough chelation solution to check every air filter in ImpSec HQ. Illyan exchanged abstracted greetings with his old secretary. After a few moments, General Haroche excused himself. Illyan did not look up.

Miles watched out of the corner of his eye as Haroche exited into the corridor. Hook set, yes, now the line plays out. . . . He began counting in his head, timing out how a man in a suppressed panic might walk to one room, then another. He motioned Weddell to desist with his spray; when he reached one hundred, he spoke. "All right, gentlemen. If you will follow me one more time. Quietly, please."

He led them out into the corridor and turned left, and right again at the second intersection. In the middle of that hallway, he met the commodore who had taken over Domestic Affairs from Haroche.