You're a pretty girl… These days, there are very few men who would say something like that to a twelve-year-old. "Do you know who the man was?" I asked. "Had you ever seen him before?"

She shook her head.

"What did he look like?"

Silence; a breath. "Big."

"Big like me? Tall?"

"Yeah…um…yeah. But big like this, too." She stretched out her arms; the glass wobbled precariously.

"A fat man?"

Jessica giggled, a sharp, nervous sound. "Yeah."

"What was he wearing?"

"A, a tracksuit. A dark-blue one." She glanced at Rosalind, who nodded encouragingly.

Shit, I thought. My heart was speeding. "What was his hair like?"

"No. He didn't have hair."

I made a quick, fervent mental apology to Damien: apparently he hadn't, after all, just been telling us what we wanted to hear. "Was he old? Young?"

"Like you."

"When did this happen?"

Jessica's lips parted, moved soundlessly. "Huh?"

"When did you and Katy meet the man? Was it just a few days before Katy went away? Or a few weeks? Or a long time ago?"

I was trying to be sensitive, but she flinched. "Katy didn't go away," she said. "Katy got killed." Her eyes were starting to lose focus. Rosalind shot me a reproachful look.

"Yes," I said, as gently as I could, "she did. So it's very important for you to try and remember when you saw this man, so we can find out if he's the one who killed her. Can you do that?"

Jessica's mouth fell a little open. Her eyes were unreachable, gone.

"She told me," Rosalind said softly, over her head, "that this happened a week or two before…" She swallowed. "She's not sure of the exact date."

I nodded. "Thank you so much, Jessica," I said. "You've been very brave. Do you think you would know this man if you saw him again?"

Nothing; not a flicker. The sugar packet hung loosely in her curled fingers. "I think we should go," Rosalind said, looking worriedly from Jessica to her watch.

I watched from the window as they walked away down the street: Rosalind's decisive little steps and the delicate sway of her hips, Jessica dragging along behind her by the hand. I looked at the back of Jessica's silky bent head and thought of those old stories where one twin is hurt and the other, miles away, feels the pain. I wondered if there had been a moment, during that giggly girls' night at Auntie Vera's, when she had made some small, unnoticed sound; if all the answers we wanted were locked away behind the strange dark gateways of her mind.

You're the perfect person for this case, Rosalind had said to me, and the words were still ringing in my head as I watched her go. Even now, I wonder whether subsequent events proved her completely right or utterly and horribly wrong, and what criteria one could possibly use to tell the difference.

Over the next few days, I spent practically every waking moment searching for the mystery tracksuit. Seven guys around Knocknaree matched the description, such as it was-tall, heavily built, thirties, bald or skinheaded. One of them had a minor record, left over from his wild youth: possession of hash, indecent exposure-my heart skipped a beat when I saw that, but all he had done was take a leak down a laneway just as an earnest young cop was passing. Two said they might have been going into the estate on their way home from work at about the time Damien had given us, but they weren't sure.

None of them would admit to having talked to Katy; all of them had alibis, more or less, for the night of her death; none of them had a dancing daughter with a broken leg, or anything like a motive, as far as I could discover. I got photos and did lineups for Damien and Jessica, but they both gave the array of photographs the same dazed, hunted look. Damien finally said he didn't think any of them was the man he had seen, while Jessica pointed tentatively to a different picture every time she was asked and finally turned catatonic on me again. I had a couple of floaters go door-to-door, asking everyone in the estate whether they had had a visitor matching the description: nothing.

A couple of the alibis were uncorroborated. One guy claimed he had been online till almost three in the morning, on a bikers' forum, discussing the maintenance of classic Kawasakis. Another said he had been on a date in town, missed the 12:30 night bus and waited for the 2:00 one in Supermac's. I stuck their photos up on the whiteboard and set about trying to break the alibis, but every time I looked at them I got the same feeling, a specific and unsettling feeling that I was starting to associate with this whole case: the sensation of another will meeting mine at every turn, something sly and obstinate, with reasons of its own.

Sam was the only one getting anywhere. He was out of the office a lot, interviewing people-county council members, he said, surveyors, farmers, members of Move the Motorway. At our dinners he was vague about where all this was taking him: "I'll show you in a few days," he said, "when it starts to make sense." I sneaked a glance at his notes once, when he went to the bathroom and left them on his desk: diagrams and shorthand and little sketches in the margins, meticulous and indecipherable.

Then on Tuesday-a muggy, petulant, drizzly morning, Cassie and I grimly going through the floaters' door-to-door reports again in case we had missed something-he came in with a big roll of paper, the heavy kind that children use to make valentines and Christmas decorations in school. "Right," he said, pulling tape out of his pocket and starting to stick the paper to the wall in our corner of the incident room. "Here's what I've been doing all this time."

It was a huge map of Knocknaree, beautifully detailed: houses, hills, the river, the wood, the keep, all sketched in fine pen and ink with the delicate, flowing precision of a children's-book illustrator. It must have taken him hours. Cassie whistled.

"Thank you, thankyouverymuch," Sam said in a deep Elvis voice, grinning. We both abandoned our stacks of reports and went over for a closer look. Much of the map had been divided into irregular blocks, shaded in colored pencil-green, blue, red, a few in yellow. Each block held a tiny, mysterious jumble of abbreviations: Sd J. Downey-GII 11/97; rz ag-ind 8/98. I cocked an interrogative eyebrow at Sam.

"I'll explain it now." He bit off another piece of tape and secured the last corner. Cassie and I sat on the edge of the table, where we were close enough to see the details.

"OK. See this?" Sam pointed to two parallel dashed lines curving across the map, cutting through the wood and the dig. "That's where the motorway's going to be. The government announced the plans in March of 2000 and bought the land off local farmers over the next year, under a compulsory purchase order. Nothing dodgy there."

"Well," Cassie said. "Depending on your point of view."

"Shhh," I told her. "Just look at the pretty picture."

"Ah, you know what I mean," Sam said. "Nothing you wouldn't expect. Where it gets interesting is the land around the motorway. That was all agricultural land, too, up until late 1995. Then, bit by bit, over the next four years, it started getting bought up and rezoned, from agricultural to industrial and residential."

"By clairvoyants who knew where the motorway was going to be, five years before it was announced," I said.

"That's not actually that dodgy either," Sam said. "There was talk about a motorway coming into Dublin from the southwest-I've found newspaper articles-starting in about 1994, when the economic boom kicked in. I talked to a couple of surveyors, and they said this was the most obvious route for a motorway, because of topography and settlement patterns and a load of other things. I didn't understand the whole of it, but that's what they said. There's no reason why property developers couldn't have done the same thing-got wind of the motorway and hired surveyors to tell them where it was likely to go."