A law enforcement official escorts Cosmo DiNardo to a vehicle Thursday, July 13, 2017, in Doylestown, Pa. Lawyer Paul Lang, a defense attorney for DiNardo, said Thursday that his client has admitted killing the four men who went missing last week and told authorities the location of the bodies. Lang says prosecutors agreed to take the death penalty off the table in return for DiNardo's cooperation. (AP Photo/Matt Rourke)

A law enforcement official escorts Cosmo DiNardo to a vehicle Thursday, July 13, 2017, in Doylestown, Pa. Lawyer Paul Lang, a defense attorney for DiNardo, said Thursday that his client has admitted killing the four men who went missing last week and told authorities the location of the bodies. Lang says prosecutors agreed to take the death penalty off the table in return for DiNardo's cooperation. (AP Photo/Matt Rourke)

DOYLESTOWN, Pa. (AP) — A drug dealer has confessed to selling four young men marijuana before killing them and 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 publicly discuss details of the case. The person said a co-conspirator was involved in the shooting deaths of three of the men, who had been reported missing.

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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, and had told investigators where their bodies were.

DiNardo, 20, 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 Doylestown courthouse.

The person with firsthand knowledge of DiNardo’s confession said the men were shot in the head or the back after DiNardo felt cheated or threatened during three drug transactions. DiNardo sold quarter-pound (113-gram) 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.

DiNardo said one victim was killed July 5 and the other three were killed July 7, the person said. The remains of the last three killed were reported discovered Wednesday by investigators, and DiNardo agreed to tell investigators the separate location of the first victim’s body, 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 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 DiNardo’s 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 Thursday and planned a briefing Friday.

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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 the 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 early 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 officially identified.

Weintraub said the victims’ families were holding up “remarkably well.”

DiNardo, whose parents own the Solebury 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 early 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, told philly.com that DiNardo routinely sold guns and had talked “about weird things like killing people and having people killed.”

DiNardo’s father initially 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 of his mental health problems.

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 DiNardo’s re-arrest.

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Izaguirre reported from Philadelphia.

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This story has been corrected to show Patrick’s middle name is Taro, not Tar, as prosecutors had reported.