Candles guttered in an unceasing draught; the air smelled of damp and incense and dying flowers. I was light-headed-I had forgotten to eat breakfast-and the whole scene had the glass-covered quality of memory. It took me awhile to realize that this was, in fact, for good reason: I had attended Mass here every Sunday for twelve years, had quite possibly sat through a memorial service for Peter and Jamie in one of the cheap wooden pews. Cassie blew into her hands, surreptitiously, to warm them.

The priest was very young and solemn, trying painfully hard to rise to the occasion with his frail seminary arsenal of clichés. A choir of pale little girls in school uniforms-Katy's schoolmates; I recognized some of the faces-huddled shoulder to shoulder, sharing hymn sheets. The hymns had been chosen to offer comfort, but the voices were thin and uncertain and a few girls kept breaking down. "Be not afraid, I go before you always; come, follow me…"

Simone Cameron caught my eye on her way back from Communion and gave me a tiny stiff nod; her golden eyes were bloodshot, monstrous. The family filed out of their pew one by one and laid mementoes on the coffin: a book from Margaret, a stuffed toy shaped like a ginger cat from Jessica, from Jonathan the pencil drawing that had hung above Katy's bed. Last of all Rosalind knelt down and placed a pair of small pink ballet shoes, bound together by their ribbons, on the lid. She stroked the shoes gently and then bent her head onto the coffin and sobbed, her warm brown ringlets tumbling over the white and gold. A faint, inhuman wail rose from somewhere in the front pew.

Outside, the sky was gray-white and wind was whipping leaves off the trees in the churchyard. Reporters were leaning over the railings, cameras firing in swift bursts. We found a discreet corner and scanned the area and the crowd, but unsurprisingly no one rang any alarm bells. "Some turnout," Sam said quietly. He was the only one of us who had gone up for Communion. "Let's get film off some of these lads tomorrow, check if anyone's here who shouldn't be."

"He's not here," Cassie said. She dug her hands into her jacket pockets. "Not unless he has to be. This guy won't even be reading the newspapers. He'll change the subject if anyone starts talking about the case."

Rosalind, moving slowly down the church steps with a handkerchief pressed to her mouth, raised her head and saw us. She shook off the supporting arms and ran across the grass, long black dress fluttering in the wind. "Detective Ryan…" She caught my hand in both of hers and raised a tear-stained face to me. "I can't bear it. You have to catch the man who did this to my sister."

"Rosalind!" Jonathan called hoarsely, somewhere, but she didn't look away. Her hands were long-fingered and soft and very cold. "We'll do everything we can," I said. "Will you come in and talk to me tomorrow?"

"I'll try. I'm sorry about Friday, but I couldn't…" She glanced quickly over her shoulder. "I couldn't get away. Please find him, Detective Ryan-please…"

I felt, more than heard, the spatter of the cameras. One of the photos-Rosalind's anguished, upturned profile, an unflattering shot of me with my mouth open-made it onto the front page of a tabloid the next morning, with PLEASE GIVE MY SISTER JUSTICE below it in letters an inch high, and Quigley gave me grief about it all week.

In the first two weeks of Operation Vestal we did everything you can think of, everything. Between us and the floaters and the local uniforms, we talked to everyone who lived within a four-mile radius of Knocknaree and anyone who had ever known Katy. There was one diagnosed schizophrenic on the estate, but he had never hurt anyone in his life, even when he was off his meds, which he hadn't been in three years. We checked out every Mass card the Devlins got and tracked down every person who'd contributed towards Katy's fees, and set up surveillance to see who brought flowers to lay on the altar stone.

We interviewed Katy's best friends-Christina Murphy, Elisabeth McGinnis, Marianne Casey: red-eyed, shaky, brave little girls, with no useful information to offer, but I found them disconcerting nevertheless. I have no time for people who sigh about how quickly children grow up nowadays (my grandparents, after all, were working full-time by sixteen, which I think trumps any number of body piercings in the adulthood stakes), but all the same: Katy's friends had a poised, savvy awareness of the outside world that jarred with the happy animal oblivion I remembered enjoying at that age. "We wondered if Jessica had a learning disability, maybe," Christina said, sounding about thirty, "but we didn't want to ask. Did…I mean, was it a pedophile that killed Katy?"

The answer to this appeared to be no. In spite of Cassie's feeling that this hadn't really been a sex crime, we checked out every convicted sex offender in south Dublin, as well as plenty whom we've never been able to convict, and we spent hours with the guys who have the thankless job of tracking and trapping pedophiles online. The guy we mostly talked to was called Carl. He was young and skinny, with a lined white face, and he told us that after eight months on this job he was already thinking of quitting: he had two kids under seven, he said, and he couldn't look at them the same way any more, he felt too dirty to hug them good night after a day of doing what he did.

The network, as Carl called it, was buzzing with speculation and titillation about Katy Devlin-I'll spare you the details-and we read through hundreds of pages of chat transcripts, dispatches from a dark and alien world, but we came up empty. One guy seemed to empathize a little too strongly with Katy's killer ("I think he just LOVED HER TO MUCH she didn't understand so he got UPSET"), but when she died he had been online, discussing the relative physical merits of East Asian versus European little girls. Cassie and I both got very drunk that night.

Sophie's gang went over the Devlins' house with a fine-tooth comb-ostensibly collecting fibers and so on, for elimination purposes, but they reported back that they had found no bloodstains and nothing matching Cooper's description of the rape weapon. I pulled financial records: the Devlins lived modestly (one family holiday, to Crete, four years earlier on a credit-union loan; Katy's ballet lessons and Rosalind's violin; a '99 Toyota) and had almost no savings, but they weren't in any debt, their mortgage was almost paid off, they had never even fallen into arrears on their phone bill. There was no dodgy activity on their bank account and no insurance policy on Katy's life; there was nothing.

The tip line got a record number of calls, an incredible percentage of which were utterly useless: the people whose neighbors looked funny and refused to join the Residents' Association, the people who had seen sinister men hanging around halfway across the country, the usual assorted whackjobs who had had visions of the murder, the other set of whackjobs explaining at length how this was God's judgment on our sinful society. Cassie and I spent a full morning on one guy who rang up to tell us that God had punished Katy for her immodesty in exhibiting herself, dressed only in a leotard, to thousands of Irish Times readers. We had high hopes of him, actually-he refused to talk to Cassie, on the grounds that women shouldn't be working and that her jeans were also immodest (the objective standard for female modesty, he informed me vehemently, was Our Lady of Fatima). But his alibi was impeccable: he had spent the Monday night in the minuscule red-light district off Baggot Street, drunk as a skunk, shrieking fire and brimstone at the hookers and writing down their clients' plate numbers and getting forcibly removed by the pimps and starting all over again, until the cops had finally thrown him in a cell to sleep it off at around four in the morning. Apparently this happened every few weeks or so; everyone concerned knew the drill and was happy to confirm it, with the odd pungent remark about the guy's probable sexual proclivities.