I said, "Jesus Christ." Katy Devlin had died on this floor. We had sat in this shed and interviewed the killer, smack bang on the scene of the crime.

"No chance that's bleach or something," said Cassie. Luminol gives false positives for anything from household bleach through copper, but we both knew Sophie wouldn't have called us in here until she was sure.

"We've swabbed," Sophie said briefly. I could hear the dirty look in her voice. "Blood."

Deep down, I think I had stopped believing in this moment. I had thought an awful lot about Kiernan, over the past few weeks: Kiernan, with his cozy seaside retirement and his haunted dreams. Only the luckiest of detectives makes it through a whole career without at least one of these cases, and some traitor part of me had insisted from the start that Operation Vestal-the last one in the world I would have chosen-was going to be mine. It took a strange, almost painful adjustment of focus to understand that our guy was no longer a faceless archetype, coalesced out of collective nightmare for one deed and then dissolved back into darkness; he was sitting in the canteen, just a few yards away, wearing muddy Docs and drinking tea under O'Gorman's fishy eye.

"There you go," Sophie said. She straightened up and switched on the overhead light. I blinked at the bland, innocent floor.

"Look," said Cassie. I followed the tilt of her chin: on one of the bottom shelves was a plastic bag stuffed with more plastic bags, the big, clear, heavy kind the archaeologists used for storing pottery. "If the trowel was a weapon of opportunity…"

"Oh, for fuck's sake," Sophie said. "We're going to have to test every bag in this whole bloody place."

The windowpanes rattled and there was a sudden, wild thrumming on the roof of the shed: it had started to rain.

20

It rained hard all the rest of the day, the kind of thick, endless rain that can soak you to the skin as you run the few yards to your car. Every now and then lightning forked over the dark hills, and a distant rumble of thunder reached us. We left the Bureau gang to finish processing the scenes and took Hunt, Mark, Damien and, on the off-chance, a deeply aggrieved Sean ("I thought we were partners here!") back to work with us. We found them an interview room each and started rechecking their alibis.

Sean was easy to eliminate. He shared a flat in Rathmines with three other guys, all of whom remembered, to some extent, the night Katy had died: it had been one of the guys' birthday and they had had a party, at which Sean had DJed till four in the morning, then thrown up on someone's girlfriend's boots and passed out on the sofa. At least thirty witnesses could vouch for both his whereabouts and his tastes in music.

The other three were less straightforward. Hunt's alibi was his wife, Mark's was Mel; Damien lived in Rathfarnham with his widowed mother, who went to bed early but was positive he couldn't have left the house without waking her. These are the kind of alibis detectives hate, the thin, mulish kind that can wreck a case. I could tell you about a dozen cases where we know exactly whodunit, how and where and when, but there is absolutely nothing we can do about it because the guy's mammy swears he was tucked up on the sofa watching The Late Late Show.

"Right," O'Kelly said, in the incident room, after we had taken Sean's statement and sent him home (he had forgiven me for my treachery and offered me a farewell high five; he wanted to know if he could sell his story to the papers, but I told him if he did I would personally raid his flat for drugs every night until he was thirty). "One down, two to go. Place your bets, lads: who do ye fancy?" He was in a much better mood with us, now that he knew we had a suspect in one of the interview rooms, even if we weren't sure which one.

"Damien," Cassie said. "He fits the MO, bang on."

"Mark's admitted he was at the scene," I said. "And he's the only one with anything like a motive."

"As far as we know." I knew what she meant, or thought I did, but I wasn't going to bring up the hired-gun theory, not in front of either O'Kelly or Sam. "And I can't see him doing it."

"I'm aware of that. I can."

Cassie rolled her eyes, which I actually found slightly comforting: a small savage part of me had expected her to flinch.

"O'Neill?" O'Kelly asked.

"Damien," Sam said. "I brought them all a cup of tea. He's the only one picked his up with his left hand."

After a startled second, Cassie and I started to laugh. The joke was on us-I, at any rate, had forgotten all about the left-handed thing-but we were both wound tight and giddy, and we couldn't stop. Sam grinned and shrugged, pleased at the reaction. "I don't know what ye two are laughing about," O'Kelly said gruffly, but his mouth was twitching, too. "You should've spotted that yourselves. All this jibber-jabber about MOs…" I was laughing too hard, my face going red and my eyes watering. I bit down on my lip to stop myself.

"Oh, God," said Cassie, taking a deep breath. "Sam, what would we do without you?"

"That's enough fun and games," O'Kelly said. "You two take Damien Donnelly. O'Neill, get Sweeney and have another go at Hanly, and I'll find a few of the lads to talk to Hunt and the alibi witnesses. And, Ryan, Maddox, O'Neill-we need a confession. Don't fuck this up. Ándele." He scraped back his chair with an ear-splitting screech and left.

"Ándele?" said Cassie. She looked perilously near to another bout of the giggles.

"Well done, lads," Sam said. He held out a hand to each of us; his grip was strong and warm and solid. "Good luck."

"If Andrews hired one of them," I said, when Sam had gone to find Sweeney, and Cassie and I were alone in the incident room, "this is going to be the mess of the century."

Cassie raised one eyebrow noncommittally. She finished her coffee: it was going to be a very long day, we had all been spiking ourselves up on caffeine.

"How do you want to do this?" I asked.

"You head it up. He thinks of women as the source of sympathy and approval; I'll pat him on the head now and then. He's intimidated by men, so go easy: if you push him too hard, he'll freeze up and want to leave. Just take your time, and guilt-trip him. I still think he was in two minds about the whole thing from the start, and I bet he feels terrible about it. If we play to his conscience, it's only a matter of time before he goes to pieces."

"Let's do it," I said, and we shook our clothes straight and smoothed down our hair and walked, shoulder to shoulder, down the corridor towards the interview room.

It was our last partnership. I wish I could show you how an interrogation can have its own beauty, shining and cruel as that of a bullfight; how in defiance of the crudest topic or the most moronic suspect it keeps inviolate its own taut, honed grace, its own irresistible and blood-stirring rhythms; how the great pairs of detectives know each other's every thought as surely as lifelong ballet partners in a pas de deux. I never knew and never will whether either Cassie or I was a great detective, though I suspect not, but I know this: we made a team worthy of bard-songs and history books. This was our last and greatest dance together, danced in a tiny interview room with darkness outside and rain falling soft and relentless on the roof, for no audience but the doomed and the dead.

Damien was huddled in his chair, shoulders rigid, his cup of tea steaming away ignored on the table. When I cautioned him, he stared at me as if I were speaking Urdu.

The month since Katy's death hadn't been kind to him. He was wearing khaki combats and a baggy gray sweatshirt, but I could see that he had lost weight, and it made him seem gangly and somehow shorter than he actually was. The boy-band prettiness was looking a little ragged around the edges-purplish bags under his eyes, a vertical crease starting to form between his eyebrows; the youthful bloom that should have lasted him another few years was fading fast. The change was subtle enough that I hadn't noticed it back on the dig, but now it gave me pause.