Because the cause of the towers' collapse is known, said the Police Department's chief of detectives, William Allee, the search for evidence is secondary to the effort to ''find something that belongs to somebody.'' He wants families to know: ''These truckloads of destruction are not just being brought here and dumped in a big hole. There is a process. There is a protocol.''

Yesterday morning, investigators found a hand, which may be identifiable through fingerprints. And Chief Allee said they found a ''pile of mud'' that turned out to be an American flag. He washed it and hung it in the work area.

It is the hunt for the solitary item or human fragment -- the possible key to an unknown family's peace -- that energizes the likes of Efram Negron, a narcotics detective. He normally works in the South Bronx, but on Friday he was sitting on a concrete block, waiting for a backhoe to lay out another pile of debris to be raked. Through a respirator that made his voice seem far away, he said, ''I'll come up here for a year, or two years, it doesn't matter.''

Sitting nearby, Joseph Pirrello, a 68-year-old retired police lieutenant who would normally be playing golf on a Friday morning, agreed. Arrayed before him were rakes and shovels, and beyond them, tons of unexamined rubble: piping and plastic and cement and steel and paper, all improbably fused together and coated with the gray powder of pulverized concrete. He repeated the message that superiors on this assignment use to rally their already-motivated troops:

''Somebody's got to do it.''

It is something that has never been done. Without precedent to guide them, through trial and error, the Police Department and various other government agencies have developed a fairly sophisticated process to refine crudely jumbled debris. On a 135-acre, waste-made plateau rising 180 feet in central Staten Island -- where there had been nothing but a foul wind and a glorious view -- they have built a village to sort the evidence of a singular crime.

''The only thing that was up here was nothing,'' said Deputy Inspector James Luongo. Before the attack on Sept. 11, he was the executive officer of the Fugitive Enforcement Division in Brooklyn; after the 11th, he became the incident commander of a round-the-clock evidence-recovery operation that is known, simply, as The Hill.