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 today.

The person spoke to The Associated Press on the condition of anonymity because he wasn’t authorized to publicly discuss details of the case. 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 12½-foot-deep grave on a farm. But what sort of evil befell them, and why, had remained shrouded in secrecy.

The prosecutor, who has held twice-daily briefings, made it clear Thursday he knew a lot more than he was saying, citing the need to protect the investigation. That only added to the speculation and rumors before DiNardo’s confession.

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.

DiNardo, whose parents own the farm, remained in custody on $5 million cash bail, accused so far only of a gun possession charge and trying to sell one of the victims’ cars.

Police were back at the farm Thursday, digging away in the dust and the 90-degree-plus heat and using plywood to shore up the deep, tent-covered trench that they excavated at the spot where Weintraub said dogs smelled “these poor boys 12½ feet below the ground.”

For days, TV news helicopters trained their cameras on the excavation, creating an unsettling racket but allowing the public to follow the forensic work from their office computers. Viewers could watch investigators haul up buckets of dirt and sift it through handheld screens in what looked like an archaeological dig.

When the prosecutor held a dramatic midnight Wednesday news conference to announce the discovery of remains, Claire Vandenberg, of neighboring New Hope, gathered around a TV with a group of friends to hear developments on what she said is “all we talk about.”

“It seemed almost like a horror film or something, just unraveling before our eyes,” she said.

Eric Beitz, who said he had hung out with DiNardo in recent weeks, told philly.com that DiNardo routinely sold guns and on multiple occasions had talked “about weird things like killing people and having people killed.”

DiNardo’s father bailed him out, but he was jailed again later in the week on the stolen-car charges, and bail was set much higher, after a prosecutor said he was a danger to the community because he had been diagnosed as schizophrenic.

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. Patrick and DiNardo had attended the same Catholic high school for boys.

It was the discovery of Meo’s car on a DiNardo family property a half-mile from the farm that led to Cosmo DiNardo’s re-arrest.