The police had been investigating the disappearance of four men in Pennsylvania for only a week when prosecutors sprang a deal on their chief suspect: Confess to murder and help the investigation, and avoid the death penalty.

Cosmo DiNardo, 20, a college dropout and scion of an affluent family in Bucks County, Pa., quickly said yes to the bargain. He and an accomplice were charged on Friday with multiple counts of criminal homicide.

In so swiftly wrapping up the case, which transfixed the Philadelphia region, the district attorney of Bucks County, Matthew D. Weintraub, faced questions about whether he had made the right call in taking the most severe punishment for horrible crimes off the table. Experts in death penalty law said the agreement was especially notable for its speed. But the father of one of the young men found dead said on Monday that family members of all of the victims supported it.

Mark Potash, whose son Mark Sturgis, 22, was killed, said Mr. Weintraub came to the family members, who for days had stood vigil while investigators, aided by cadaver-sniffing dogs, excavated a burial pit at a farm in Solebury Township. At the time, the bodies of Mr. Sturgis and two other victims, Dean Finocchiaro, 19, and Thomas Meo, 21, had been found, but one victim was unaccounted for. The prosecutor said he needed Mr. DiNardo’s cooperation.