Cooper was enjoying this way too much. “I don’t need round-the-clock protection,” I said. “If this guy was obsessed with me, he wouldn’t have stopped at one blow, any more than he would’ve if he was obsessed with Lexie. Everybody relax.”

“Right,” Sam said, after a moment. He didn’t sound happy. “I think that’s the lot.” He sat down, hard, and pulled his chair up to his desk.

“She wasn’t killed for her money, anyway,” Frank said. “The five of them pool most of their funds-a hundred quid a week each into a kitty, to pay for food, petrol, bills, doing up the house, all the rest of it. On her income, that didn’t leave much. She had eighty-eight quid in her bank account.”

“What do you think?” Sam asked me.

He meant from a profiling angle. Profiling is nowhere near foolproof and I don’t actually have much of a clue what I’m doing anyway, but as far as I could see, everything said she had been killed by someone she knew, someone with a hair-trigger temper rather than a well-nursed grudge. The obvious answer was either the kid’s father or one of the housemates, or both.

But if I said that, then this meeting was over, at least as far as I was concerned; Sam would blow every gasket at the thought of me sharing a house with the odds-on favorites. And I didn’t want that. I tried to tell myself it was because I wanted to make the decision, not have Sam make it for me, but I knew: this was working on me, this room and this company and this conversation, pressing subtly just like Frank had known they would. Nothing in this world takes over your blood like a murder case, nothing demands you, mind and body, with such a huge and blazing and irresistible voice. It had been months since I had worked like this, concentrated like this on fitting together evidence and patterns and theories, and all of a sudden it felt like years.

“I’d go with door number two,” I said, finally. “Someone who knew her as Lexie Madison.”

“If that’s where we’re focusing,” Sam said, “her housemates were the last ones to see her alive, and they’re the ones were closest to her. That puts them front and center.”

Frank shook his head. “I’m not so sure. She was wearing her coat, and it wasn’t put on her after she died-there’s a slit in the front right side, perfect match to the wound. To me, that says she was out of the house, away from the housemates, when she got stabbed.”

“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.

“Don’t look at me,” O’Kelly said. “Your case, your call.” If this thing went splat, and he obviously thought it would, he wanted to be well out of range.

All three of them were starting to get right up my nose. “Remember me?” I inquired. “You might want to start trying to convince me, too, Frank, because I’d say this was at least partly my call.”

“You’ll go where you’re sent,” said O’Kelly.

“It is, of course,” Frank told me reproachfully. “I’m getting to you. I felt it would be polite to start by discussing matters with Detective O’Neill, what with this being a joint investigation and all. Am I wrong?”

This is why joint investigations are from hell: nobody is ever quite sure who the big boss is, and nobody wants to find out. Officially, Sam and Frank were supposed to agree on any major decisions, but if it came to the crunch, anything to do with undercover was Frank’s call. Sam could probably override him, since this had started out as his investigation, but not without an awful lot of string-pulling and a damn good reason. Frank was making sure-I felt it would be polite-that Sam remembered that. “You’re dead right,” I said. “Just remember, you need to discuss matters with me, too. So far, I haven’t heard anything very convincing.”

“How long are we talking about?” Sam asked. He was asking Frank, but his eyes were on me, and the look in them startled me: they were intent and very grave, almost sad. That was the second when I realized Sam was going to say yes.

Frank saw it too; his voice didn’t change, but his back had straightened and there was a new spark in his face, something alert and predatory. “Not long. A month, max. It’s not like we’re investigating organized crime and we could need someone on the inside for years. If this doesn’t pay off inside a few weeks, then it’s not going to.”

“She’d have backup.”

“Twenty-four-hour.”

“If there’s any indication of danger-”

“We’ll pull Detective Maddox out straightaway, or go in and get her if we need to. Same if you develop information that means she’s no longer necessary to the investigation: we’ll have her out that same day.”

“So I’d better get cracking,” Sam said quietly, on a long breath. “OK: if Detective Maddox wants to do it, then we’ll do it. On condition that I’m kept fully informed of all developments. No exceptions.”

“Beautiful,” Frank said, sliding off his chair fast, before Sam could change his mind. “You won’t regret it. Hang on, Cassie-before you say anything, I want to show you this. I promised you videos, and I’m a man of my word.”

O’Kelly let out a snort and said something predictable about amateur porn, but I barely heard him. Frank fished around in his big black knapsack, waved a DVD labeled in marker scrawl at me and shoved it into the squad room’s cheapo DVD player.