DiNardo has told investigators where all of the bodies are, Lang said.
In exchange, authorities took the possibility of the death penalty off the table.
DiNardo has "deep remorse" for what he did, lawyer Paul Lang said. He made the statement to reporters on Thursday evening.
Human remains have been found amid an extensive search on farmland owned by DiNardo's family.
A body found there has been identified as that of 19-year-old Dean Finocchiaro.
Finocchiaro, 22-year-old Mark Sturgis, 21-year-old Tom Meo and 19-year-old Jimi Patrick have been missing since last week.
THIS IS A BREAKING NEWS UPDATE. PREVIOUS STORY BELOW
Police on Thursday returned to a large Pennsylvania farm to resume digging and sifting through a deep common grave where multiple remains were discovered, including one identified as one of four men missing since last week.
Bucks County District Attorney Matthew Weintraub said at a midnight news conference that one victim has been identified as 19-year-old Dean Finocchiaro.
VIDEO: Chopper 6 HD over the investigation site Thursday
RAW VIDEO: Bucks Co. investigating
Authorities were not yet ready to identify any other remains, Weintraub said at a news conference on Thursday morning.
Weintraub says they found all the remains in a 12.5-foot-deep common grave on a sprawling farm in Solebury Township.
Crews continue to work around the clock.
"They are side-by-side, and combing through vast fields, and down 12 feet deep in a hole that is getting deeper by the minute," Weintraub said.
Authorities said they are looking at pursuing homicide charges against 20-year-old Cosmo DiNardo, who was taken into custody on Wednesday on vehicle theft charges.
"This is a homicide; make no mistake about it. We just don't know how many homicides," Weintraub said.
VIDEO: D.A. Weintraub midnight news conference Thursday
Bucks Co. D.A. holds news conference on missing men
Finocchiaro, 22-year-old Mark Sturgis, 21-year-old Tom Meo and 19-year-old Jimi Patrick have been missing since last week.
Authorities have vowed to "bring each and every one of these lost boys home to their families, one way or another."
Patrick was a year behind DiNardo at a Catholic high school for boys, the Associated Press reports. It's unclear how well the four knew DiNardo, if at all.
DiNardo's arrest on Wednesday was his second arrest of the week. The new charges allege that he tried to sell a car belonging to Meo a day after Meo was last seen.
DiNardo's being held on $5 million full cash bail. The judge said it was the highest bail she has ever set.
DiNardo has not been called a suspect in the disappearance of the men or the death of Finocchiaro.
RAW VIDEO: Cosmo DiNardo arrives at Bucks Co. prison
VIDEO: Cosmo DiNardo arrives at Bucks County prison
Booking photo of Cosmo DiNardo from Wednesday's arrest
On Thursday morning, a grand jury is scheduled to meet at the Bucks County Courthouse in Doylestown. The parents of Cosmo DiNardo have been subpoenaed to appear, sources tell Action News.
VIDEO: Grand jury convened in Bucks County
Grand Jury convened in Bucks County
Attorney Fortunato Perri Jr. is representing Cosmo DiNardo. On Wednesday, he released a statement on behalf of his parents that reads:
"As parents, Mr. and Mrs. DiNardo sympathize with the parents and families of the missing young men and they are cooperating in every way possible with the investigation being conducted by law enforcement."
The DiNardo's own a concrete company in Bensalem.
Cosmo DiNardo was arrested on Monday for a weapons charge from February. He was released on Tuesday on 10 percent of $1 million bail.
Weintraub detailed the vehicle theft charge for DiNardo on Wednesday afternoon.
VIDEO: D.A. Weintraub update on Wednesday afternoon
DA announces arrest of Cosmo DiNardo
According to investigators, Meo's girlfriend told investigators she last heard from him through a text message on July 7. On July 8, Meo's family reported him missing to authorities.
On July 9, Weintraub said, DiNardo allegedly tried to sell Meo's Nissan to a friend for $500. Weintraub said investigators know who that friend is, but they are withholding his identity "for potential safety."
TIMELINE: Disappearance of 4 men in Bucks County
The vehicle was later found at a property belonging to DiNardo's family on Aquetong Road in Solebury Township. The title for the vehicle was unsigned, Weintraub said, indicating there was no legal exchange of ownership.
Investigators also allegedly found a diabetic kit inside the car. Meo is diabetic, Weintraub said, and is known to carry that "life-saving" kit with him.
DiNardo is charged with theft and receiving stolen property.
"I don't know it, but it's possible that this charge will be enveloped in a larger set of circumstances," Weintraub said. "I don't want to comment on that prematurely, though."
Like Meo, Sturgis and Finocchiaro were last seen on July 7. Patrick was last seen on July 5.
According to an affidavit of probable cause from the February incident, DiNardo was accused of possessing a shotgun and ammunition despite a history of mental illness that includes an involuntary commitment.
VIDEO: Chopper 6 over search scene on Wednesday
Chopper 6 HD over search for missing men
Sources tell ABC News that a ping from one of the missing men's cellphones led investigators to the farmland.
Investigators continue to ask the public for tips.
Anyone with information is asked to contact FBI investigators at 1-800-CALL-FBI, or go to tips.fbi.gov.