"I'm not eliminating them yet," Sam said. "I don't know why any of them would want to stab her, I don't know why they'd do it outside the house, all I know is that on this job the obvious answer is mostly the one you're after--and any way you look at it, they're the obvious answers. Unless we find a witness who saw her alive and well after she left that house, I'm keeping them in."

Frank shrugged. "Fair enough. Say it's one of the housemates: they're sticking together like glue, they've been interviewed for hours without batting an eyelid, the chances of us breaking their story are virtually nil. Or say it's an outsider: we don't have the foggiest clue who he is, how he knew Lexie or where to start looking for him. There are some cases that just plain can't be broken from the outside. That's why Undercover exists. Which brings me back nicely to my alternative tack."

"Throwing a detective into the middle of a bunch of murder suspects," Sam said.

"Just as a rule," Frank told him, with an amused little lift of one eyebrow, "we don't send undercovers to investigate holy innocents. Being surrounded by criminals is what we tend to do."

"And we're talking IRA, gangsters, dealers," O'Kelly said. "This is a bunch of fucking students. Even Maddox can probably handle them."

"Exactly," Sam said. "Exactly. Undercover investigates organized crime: drugs, gangs. They don't go in on your run-of-the-mill murder. Why do we need them on this one?"

"From a murder detective," Frank said, concerned, "that amazes me. Are you saying that this girl's life is worth less than a K of heroin?"

"No," Sam said, evenly. "I'm saying there are other ways to investigate a murder."

"Like what?" Frank demanded, going in for the kill. "In the case of this particular murder, what other ways have you got? You don't have an ID on the victim"--he was leaning in towards Sam, ticking off fingers fast--"a suspect, a motive, a weapon, a primary scene, a print, a witness, trace evidence or a single good lead. Am I right?"

"It's three days into the investigation," Sam said. "Who knows what we'll--"

"Now let's look at what you have got." Frank held up one finger. "A first-rate, trained, experienced undercover who's the spitting image of the victim. That's it. Any reason why you don't want to use that?"

Sam laughed, an angry little sound, swinging his chair onto its back legs. "Why I don't want to throw her in there for shark bait?"

"She's a detective," Frank said, very gently.

"Yeah," Sam said, after a long moment. He let the front legs of his chair down again, carefully. "She is." His eyes skated away from Frank, across the squad room: empty desks in dimming corners, the explosion of scribbles and maps and Lexie on the whiteboard, me.