DOYLESTOWN, Pa. -- A drug dealer has confessed to killing four young men separately after selling them marijuana and then burning their bodies at his family's farm, a person with firsthand knowledge of his confession said Thursday.

The person spoke to The Associated Press on the condition of anonymity because he wasn't authorized to discuss details of the case publicly.

The person said a co-conspirator was involved in the killings of three of the men, who had been reported missing.

The details were provided after one of Cosmo DiNardo's lawyers said Thursday that DiNardo had confessed to murdering the missing men, who included a Maryland college student.

DiNardo, 20, confessed to taking part in the four slayings and agreed to plead guilty to four first-degree murder counts, attorney Paul Lang said outside court, where DiNardo had met with investigators.

"I'm sorry," a shackled DiNardo said as he left the courthouse.

The person with firsthand knowledge of DiNardo's confession said the men were killed after DiNardo felt cheated or threatened during three drug transactions.

DiNardo sold quarter-pound quantities of marijuana for several thousand dollars and sold handguns to area residents, the person said.

"Every death was related to a purported drug transaction, and at the end of each one, there's a killing," the person said.

Authorities had charged DiNardo earlier this year with having a gun despite an involuntary mental-health commitment.

In seeking $5 million bail on a stolen-car charge this week, prosecutors said he had been diagnosed with schizophrenia.

He also suffered a head injury in an ATV accident about a year ago.

DiNardo's parents declined to comment after leaving a government building where he spent several hours with Bucks County District Attorney Matthew Weintraub and other investigators.

In exchange for the cooperation, Lang said, prosecutors were taking the death penalty off the table.

There was no immediate comment from prosecutors, but they retweeted news reports of DiNardo's confession.

The mystery of the four men's disappearances transfixed the Philadelphia area over the past week, taking a grisly turn when human remains were discovered in a 12 1/2-foot-deep grave on a farm.

The four men, all residents of Bucks County, disappeared last week. At least three knew each other.

The remains of only one, 19-year-old Dean Finocchiaro, have been identified, though authorities said other remains were found in the hole as well.

The other missing men are Mark Sturgis, 22, and Thomas Meo, 21, who worked together in construction, and Jimi Taro Patrick, 19, a student at Loyola University in Baltimore.