It was not known why the man was taken into custody. His name has not been released.
RELATED: TIMELINE: Murder of 4 men in Bucks County, Pa.
Cosmo DiNardo, 20, of Bensalem has confessed to "participating in or committing" the killings, his attorney said.
Source tells @6abc a young man from NE Philly now in custody in connex with missing Bucks men.— sarahbloomquist (@sarahbloomquist) July 14, 2017
A news conference with the Bucks County district attorney is scheduled for 2 p.m. eastern time.
DiNardo, an admitted drug dealer with a history of mental illness, said he killed the four young men after he felt cheated or threatened during three transactions and then burned their bodies at his family's farm, a person with firsthand knowledge of his confession said.
RELATED: Lawyer: Cosmo DiNardo confesses role in Pennsylvania missing men murders
The person spoke to The Associated Press on the condition of anonymity on Thursday because he was not authorized to publicly discuss details of the case against DiNardo, who was in custody.
DiNardo told authorities that a co-conspirator from Philadelphia was involved in three of the deaths, the person said.
The details were provided after one of 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 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.