She bit her lip and frowned. “Arson might be the best analogy for what Tyler and the others are up to. No question, someone can start a fire in a building with a single match. But each extra spark, each new flame, increases the likelihood of reaching flashpoint. Once you reach that point—‘critical mass’ in epidemiological terms—the fire is self-propagating. In other words, you don’t need to walk into the middle of a burning building with a can of gasoline to keep it going.”

Cohen nodded.

“And as with arson, the more extensive and damaging the fire, the harder it sometimes is to uncover the cause. The evidence gets consumed in the flames.” Lopez shot a steely glance at Cohen. “And you know what, Seth? Since no one has come forward to claim responsibility, my guess is that this is what these bastards counted on: The outbreak would rage so out of control, no one would be able to track the source back to them.”

“They never counted on you.”

“Hardly.” She broke into a glowing smile. “You’re the one who dug up Tyler’s name.”

Cohen shrugged. “He probably goes by a hundred different names.”

A musical tone rang from Lopez’s waist. She reached down and brought the cell phone to her ear. “Sure, Karen, patch him through.” Another pause. “No problem. Just hold on a sec, please.”

Lopez surprised Cohen by passing him the phone. “For you,” she said.

He brought the phone to his ear. “Seth Cohen.”

“KC,” Leetch boomed. “Is that lady with the sexy voice Dr. Lopez?”

Cohen ignored the question. “What’s up, Rome? Is Dad okay?”

“Pappy’s doing good. Saw him this morning. He wanted to know, since when did our jurisdiction extended to Seattle.” Leetch chortled. “So do I for that matter!”

“Rome, I’m on a borrowed cell phone here,” Cohen pointed out.

“Hey, how come I hear so much background noise?”

“I’m in a convertible.”

“Ah, very nice!”

“Rome …”

“Okay, okay,” Leetch said. “While you’re out cruising with some babe with the top down, I’ve been slogging away at my desk. I came across something interesting.”

“Oh?” Cohen leaned forward in his seat.

“I was working my way through Dennis Tyler’s credit card bill,” Leetch said, and Cohen knew he was going to draw out his revelation. “I finally tracked down the clerk at the Intercon in downtown Philly who checked Tyler in. Her name’s Tara Newbridge. Giggly girl but sweet.”

“She remembered Tyler?” Cohen asked impatiently.

“She said she thought he was hot.”

“Just the break we’ve been looking for,” Cohen sighed.

“Tara works only part-time for the downtown Intercon,” Leetch continued as if never interrupted. “She also works out at the Airport Intercon, picking up shifts here and there. Four days ago, Tara was working the late shift at the bell desk out at the Airport—”

“And she saw Tyler?” Cohen shot.