Not much, of course, was left after the crash except debris from the aircraft and some personal belongings. Mr. Miller said that only 8 percent of the human remains were ever recovered because the plane, roaring down at more than 570 miles per hour, exploded when it crashed. “Everything vaporized on impact,” he said.

At least some remains were recovered and matched for all 40 on board (in fact, for all 44, including the four terrorists). But the amounts were tiny — much less, even in total, than those that were unidentified…

Patrick White, vice president of Families of Flight 93, who lost his cousin Louis Nacke II in the crash, said he viewed Monday’s burial as a reunion, of sorts, of “what until now has been a disconnection, a physical separation between the ‘them’ in the three caskets and the ‘those’ who are in the ground.”

“I view it as the first — and last — reuniting of people who have a shared destiny and a now common history,” Mr. White said.