We took Mark back to the site and left him to brood darkly in the back of the car while I talked to Mel and Cassie had a quick word with their housemates. When I asked her how she'd spent Tuesday night, Mel went sunburn-red and couldn't look at me, but she said she and Mark had talked in the garden till late, ended up kissing and spent the rest of the night in his room. He had only left her once, for no more than two minutes, to go to the bathroom. "We've always got on great-the others used to take the piss out of us about it. I guess it was on the cards." She also confirmed that Mark occasionally spent the night away from the house, and that he'd told her he slept in Knocknaree wood: "I don't know if any of the others would know that, though. He's kind of private about it."

"You don't find it a little odd?"

She shrugged clumsily, rubbing at the back of her neck. "He's an intense guy. That's one of the things I like about him." God, she was young; I had a sudden urge to pat her shoulder and remind her to use protection.

The rest of the housemates told Cassie that Mark and Mel had been the last ones left in the garden Tuesday night, that they had come out of his room together the next morning and that everyone had spent the first few hours of the day, until Katy's body turned up, mercilessly giving them grief about it. They also said Mark sometimes stayed out, but they didn't know where he went. Their version of "an intense guy" ranged from "a little weird" through "a total slave driver."

We got more plasticky sandwiches from Lowry's shop and had lunch sitting on the estate wall. Mark was organizing the archaeologists into some new activity, gesturing in big militant jerks like a traffic cop. I could hear Sean complaining vociferously about something, and everyone else yelling at him to shut up and stop skiving and get a grip.

"I swear to God, Macker, if I find it on you, I'm going to shove it so far up your hole-"

"Ooh, Sean's PMS-ing."

"Have you checked up your hole?"

"Maybe the cops took it away with them, Sean, better lie low for a while."

"Get to work, Sean," Mark shouted across.

"I can't work without my fucking trowel!"

"Borrow one."

"Spare over here," someone yelled. A trowel flew spinning from hand to hand, light spiking off the blade, and Sean caught it and settled down to work, still grumbling.

"If you were twelve," Cassie said, "what would get you out here in the middle of the night?"

I thought of the faint gold circle of light, bobbing like a will-o'-the-wisp among the severed tree roots and the shards of ancient walls; the silent watcher in the wood. "We did it a couple of times," I said. "Spent the night in our tree house. This was all wood back then, right up to the road." Sleeping bags on rough boards, torch-beams close against comic books. A rustle, and the beams skidding up to cross on a pair of golden eyes, rocking wild and luminous only a few trees away; all of us yelling, and Jamie leaping up to fire a spare satsuma as the thing bounded away with a crash of leaves-

Cassie glanced at me over her juice carton. "Yeah, but you were with your mates. What would get you out here on your own?"

"Meeting someone. A dare. Possibly getting something important that I'd forgotten here. We'll talk to her friends, see if she said anything to them."

"This wasn't a random thing," Cassie said. The archaeologists had put the Scissor Sisters back on and one of her feet swung, absently, in time with the beat. "Even if it wasn't the parents. This guy didn't go out and pick up the first vulnerable kid he saw. He put a lot of planning into this. He wasn't just looking to kill a kid; he was after Katy."

"And he knew the place pretty well," I said, "if he could find the altar stone in the dark, carrying a body. It's looking more and more like a local boy." The wood was gay and sparkly in the sunlight, all birdsong and flirting leaves; I could feel the rows upon rows of identical, trim, innocuous houses ranged behind me. This fucking place, I almost said, but I didn't.

After the sandwiches we went looking for Auntie Vera and the cousins. It was a hot, still afternoon, but the estate had an eerie Marie Celeste emptiness, all the windows tightly closed and not a single kid playing; they were all inside, confused and antsy and safe under their parents' eyes, trying to eavesdrop on the adult whispers and find out what was going on.

The Foleys were an unprepossessing bunch. The fifteen-year-old settled into an armchair and folded her arms, hitching up her bust like someone's mammy, and gave us a pale, bored, supercilious stare; the ten-year-old looked like a cartoon pig and chewed gum with her mouth open, wriggling her rump on the sofa and occasionally flicking the gum out on her tongue and then back into her mouth again. Even the youngest was one of those deeply unnerving toddlers who look like bonsai adults: it had a prim, pudgy face with a beaky nose, and it stared at me from Vera's lap, its lips pursing, and then retracted its chin disapprovingly into the folds of its neck. I had a nasty conviction that, if it said anything, its voice would be a deep, forty-a-day rasp. The house smelled of cabbage. I could not fathom why on earth Rosalind and Jessica would choose to spend any time there, and the fact that they had bothered me.

With the exception of the toddler, though, they all told the same story. Rosalind and Jessica, and sometimes Katy, spent the night there every few weeks or so ("I'd love to have them more often, of course I would," said Vera, pinching tensely at a corner of a slipcover, "but I simply can't, not with my nerves, you know"); less often, Valerie and Sharon stayed with the Devlins. Nobody was sure whose idea this particular sleepover had been, although Vera thought vaguely that it might have been Margaret who suggested it. On Monday night Rosalind and Jessica had come over somewhere around half past eight, watched television and played with the baby (I couldn't imagine how; the kid had barely moved all the time we were there, it must have been like playing with a large potato), and gone to bed around eleven, sharing a camp bed in Valerie and Sharon's room.

This, apparently, was where the trouble had started: unsurprisingly, they had all four been up talking and giggling most of the night. "Now they're lovely girls, Officers, I'm not saying that, but sometimes the young people don't realize how much of a strain they can put on us old folks, isn't that right?" Vera tittered frantically and nudged the middle kid, who squirmed further away on the sofa. "I had to go in to them half a dozen times to tell them to be quiet-I can't bear noise, you know. It must have been half past two in the morning, can you imagine, before they finally went off to sleep. And by that time, of course, my nerves were in such a state that I couldn't settle at all, I had to get up and make myself a cup of tea. I didn't get a wink of sleep. I was shattered the next morning. And then when Margaret rang, sure, we were all going frantic, weren't we, girls? But I never imagined…sure, I thought she was only…" She pressed a thin, twitching hand over her mouth.

"Let's go back to the night before," Cassie said to the oldest kid. "What did you and your cousins talk about?"

The kid-Valerie, I think-rolled her eyes and pulled up her lip to show what a stupid question this was. "Stuff."

"Did you talk about Katy at all?"

"I don't know. Yeah, I guess. Rosalind was saying how brilliant it was that she was going to ballet school. I don't see what's so great about it."

"What about your aunt and uncle? Did you mention them?"