"Well, I do, of course," he said, pulling off his glasses and blinking fuzzily at me as he wiped them on his sweater. "And Mark and Damien-for the tours, you know. Just in case. People always like to see finds, don't they?"

"Yes," I said, "yes, I'm sure they do."

I went back out to the road and phoned Sam. One of the trees was dropping chestnuts, they were littered around my car, and I peeled the prickly casing off one of them and tossed it into the air while I waited for him to pick up: casual phone call, maybe setting up a date for the evening, if anyone was watching and worried; nothing important.

"O'Neill," Sam said.

"Sam, it's Rob," I said, catching the chestnut overhand. "I'm in Knocknaree, at the dig. I need you and Maddox and a few floaters down here as fast as you can, with a team from the Bureau-get Sophie Miller if you can. Make sure they bring a metal detector and someone who knows how to work it. I'll meet you at the entrance to the estate."

"Got it," Sam said, and hung up.

It would take him at least an hour to round everyone up and get out to Knocknaree. I moved my car up the hill, out of sight of the archaeologists, and sat on the hood to wait. The air smelled of dead grass and thunder. Knocknaree had closed in on itself, the far hills invisible under cloud, the wood a dark illusive smear down the hillside. Enough time had passed that children were being allowed outside to play again, I heard faint high shrieks of glee or terror or both coming from inside the estate; that car alarm was still going, and somewhere a dog was barking mindlessly, frenetically, on and on and on.

Every sound wound me a notch tighter; I could feel the blood trembling in every corner of my body. My mind was still going full tilt, whirring through correlations and shards of evidence, fitting together what I needed to say to the others when they arrived. And somewhere under the adrenaline was the inexorable realization that, if I was right, then Katy Devlin's death almost certainly had nothing at all to do with what had happened to Peter and Jamie; not, at least, in any way you could enter into evidence.

I was concentrating so hard that I almost forgot what I was waiting for. When the others started arriving, I saw them with the heightened, shocked gaze of a stranger: discreet dark cars and white van pulling in with a near-silent rush, doors sliding smoothly open; the black-suited men and the faceless techs with their glittering array of tools, cool and ready as surgeons to peel back the skin of this place inch by inch and reveal the darker, seething archaeology underneath. The slamming of car doors sounded small and deadly precise, muffled by the heavy air.

"What's the story?" Sam said. He had brought Sweeney and O'Gorman and a red-haired guy whom I recognized, vaguely, from the blur of action in the incident room a few weeks earlier. I slid off the Land Rover and they moved into place around me, Sophie and her team pulling on their gloves, Cassie's thin still face over Sam's shoulder.

"The night Katy Devlin died," I said, "a trowel disappeared from the locked finds shed on the dig. The trowels they use consist of a leaf-shaped metal blade attached to a wooden handle five or six inches long, tapering inwards towards the blade, with a rounded end. This particular trowel, which is still missing, had sc burned into the handle-the initials of the owner, Sean Callaghan, who claims he forgot it in the finds shed at five thirty on Monday evening. It matches Cooper's description of the implement used to sexually assault Katy Devlin. Nobody knew it would be in the finds shed, which suggests that it was a weapon of opportunity and the shed may be our primary crime scene. Sophie, can you start there?"

"Luminol kit," Sophie said to one of her mini-mes, and he broke away from the group and clicked open the back of the van.

"Three people had keys to the finds shed," I said. "Ian Hunt, Mark Hanly and Damien Donnelly. We can't rule out Sean Callaghan, either: he could have made up the whole story about leaving the trowel there. Hunt and Hanly have cars, which means if it's one of them he might have hidden or transported the body in the trunk. Callaghan and Donnelly don't, as far as I know, so either of them would have had to hide the body fairly nearby, probably on the site. We'll have to go over the whole place with a fine-tooth comb and pray there's some evidence left. We're looking for the trowel, for a bloodstained plastic bag and for our primary and secondary crime scenes."

"Do they have keys to the other sheds, too?" Cassie asked.

"Find out," I said.

The tech was back, with the luminol kit in one hand and a roll of brown paper in the other. We looked at one another and nodded and fell into step, a swift, primed phalanx moving down the hill towards the dig.

A case breaking is like a dam breaking. Everything around you gathers itself up and moves effortlessly, unstoppably into top gear; every drop of energy you've poured into the investigation comes back to you, unleashed and gaining momentum by the second, subsuming you in its building roar. I forgot that I had never liked O'Gorman, forgot that Knocknaree wrecked my head and that I had almost blown this whole case a dozen times, almost forgot everything that had happened between me and Cassie. This, I think, is one of the things I always craved from the job: the way that, at certain moments, you can surrender everything else, lose yourself in the driving techno pulse of it and become nothing but one part of a perfectly calibrated, vital machine.

We fanned out, just in case, as we crossed the site towards the archaeologists. They gave us quick, apprehensive glances, but nobody bolted; no one even stopped working.

"Mark," I said. He was still kneeling on top of his bank; he leaped up in one fast, dangerous movement and stared at me. "I'm going to have to ask you to bring all your team into the canteen."

Mark exploded. "Jesus fuck! Have you not done enough? What are you afraid of? Even if we find the fucking Holy Grail today, your lot will still level this place on Monday morning. Could you not leave us our last few days in peace?"

For a second I almost thought he was going to come at me, and I felt Sam and O'Gorman moving in at my shoulders. "Settle down, boy," O'Gorman said threateningly.

"Don't you 'boy' me. We have till half past five on Friday and anything you want from us can wait till then, because we're going nowhere."

"Mark," Cassie said sharply, beside me. "This has nothing to do with the motorway. Here's how we're going to work this: we need you and Damien Donnelly and Sean Callaghan to come with us right now. Non-negotiable. If you quit giving us hassle, the rest of your team can keep working, under Detective Johnston's supervision. Fair enough?"

Mark glared at her for another second, but then he spat into the dust and jerked his chin at Mel, who was already moving towards him. The rest of the archaeologists stared, wide-eyed and sweating. Mark snapped instructions at Mel in an undertone, stabbing a finger at various parts of the site; then he gave her shoulder a light, unexpected squeeze and strode off towards the Portakabins, fists shoved deep in his jacket pockets. O'Gorman went after him.

"Sean," I called. "Damien." Sean bounded over eagerly and held up his hand for a high five, gave me a knowing look when I ignored it. Damien came more slowly, hitching up his combats. He looked dazed almost to the point of concussion, but coming from him this didn't exactly set my alarm bells ringing.

"We need to talk to you," I said. "We'd like you to wait in the canteen for a while, until we're ready to take you back to headquarters."