Transcript for Suspect confesses to the murders of four missing Pennsylvania men
Tonight, what appears to be a quadruple homicide rocking a Pennsylvania suburb. Police have been searching for four young men mysteriously gone missing. Two of them last seen together. But tonight the mystery appears to have been solved. After a shocking discovery on a farm that led to one man's confession. Here's ABC's Eva pilgrim. What do you have to say to these families? I'm sorry. Reporter: Tonight a stunning admission ending a more than week-long mystery in bucks county, Pennsylvania. Mr. Dinardo this evening confessed to the district attorney. He confessed to his participation and commission in the murders of four young men. Reporter: The defense attorney for 20-year-old cosmo Dinardo speaking to cameras outside the courthouse, saying his client spared the death penalty in exchange for his confession to the district attorney. When you say four young men mean the four young men who were missing? Yes. Reporter: Since last week local and federal investigators have been scouring the suburban Philadelphia community in search of four young men who vanished. We've negotiated this outcome for cosmo to be very honest, forthright, and truthful in what occurred within the last week and a half. Reporter: The search for days focusing on this 90-acre farm in solberry township owned by Dinardo's family. Officials making a grim announcement overnight, recovering the body of Dean Finocchiaro alongside other human remains. We have found human remains in an approximately 1 12 1/2-foot-deep common grave that we have painstakingly dug with a lot of care so as not to damage any potential evidence. Did he tell police where all four bodies are? Yes. I'll leave that for law enforcement. Reporter: Finocchiaro along with 22-year-old mark Sturgis and 21 saerld Thomas Meo were last seen Friday. 19-year-old Jimi Patrick has been missing since last Wednesday. I know more than I'm sharing on the relationships, and I have to keep it that way, unfortunately, to maintain the integrity of the investigation. Reporter: Officials now putting together a timeline of what they say happened. Authorities say around 4:00 P.M. Friday Sturgis went to meet Meo. Four hours later license plate readers spotted Meo's car within feet of Dinardo's truck. The next day both Meo and Sturgis failed to show up at work. Where was the sound of these gunshots coming from? Reporter: Susan Coleman says she heard gun nooir on the Dinardo family property Saturday afternoon. All of a sudden we heard this very, very loud boom, boom-boom. Then it was like boom, boom, boom, boom. Lots and lots of gunshots that were startling. And sort of reverberated. They were coming from the farm. The farm, the field right over there. They were very violent. And then I could hear in the distance like ah, ah, some sort of commotion. It was a lot of gunfire? A lot of gunfire. Reporter: On Sunday detectives discover Meo's car in this garage on the Dinardo family's property. With Meo's life-saving diabetes medications still inside. This is a homicide. Make no mistake about it. One of the things you obviouy want to determine is how did they die. And the prosecutor was very clear today that this is a homicide. So there is forensic evidence at that scene to support a homicide, which I'm going to guess could well be a firearm. Reporter: Today investigators were back at the farm delicately digging for more answers. Evacuation crews, network of victim assistance who have been there with our victims' families as they mount this round-the-clock vigil. They're side by side. They're combing through vast fields and buildings. They're down 12 foot deep in a hole that's getting deeper by the minute. They're tenderly, painstakingly, reverentially recovering the remains of people that they do not even know. Reporter: It's here amid the stifling heat where a cadaver dog led officials to a common grave. That is a very impressive hole. And clearly he was trying to bury them so deep the hope would be they would never be found. I don't understand the science behind it, but those dogs could smell these four boys 12 1/2 feet below the ground. This investigation has been some very good policework. You've had everything from cadaver dogs to cell phone evidence to video evidence. All coming back to one place, which is him. Reporter: As the community begins to mourn the loss of four young men, new and haunting images are emerging showing Dinardo pointing a pistol. Dinardo posted the photos in a group chat, telling acquaintances he wasn't worried that 19-year-old Dean Finocchiaro was still missing, suggesting instead that he was on the run. This is not some impulse homicide. He thought this through. He figured out how to do it. And in the short term he pulled it off. So that is very important in my view when it comes to when you're going to sentence him. Reporter: Dinardo was first arrested on Monday for an unrelated gun charge, then again on Wednesday after he bonded out. Officials say the 20-year-old allegedly stole a car that blongds to O belonged to one of the missing men. This is for receiving stolen property and theft of a 1996 Nissan maxima that is owned by Thomas Meo. Reporter: Dinardo then tried to sell the car for $500 the day after Meo disappeared. Today cosmo Dinardo's attorney appeared before a grand jury at the bucks county courthouse as investigators probed for information. Did he say why? That'll come out in time. I don't think it's appropriate for me out of respect for the families. Reporter: Dinardo has been known to police since he was 14 years old. A recent court document stating he suffered from mental illness and had been the subject of involuntary commitment to a mental institution. Prosecutors saying during his arraignment that he was diagnosed with schizophrenia. Can you talk about his mental health? I don't want to talk about his mental health right now. It wouldn't be a defense to just say I am mentally ill. It would have to be so mentally ill that he literally didn't understand that whahe was doing was wrong. And that's the insanity defense. If he's convicted. The only other way his mental illness could come up would be he's charged, he's convicted, and during sentencing he pleads for leniency, basically saying that he has real mental challenges. Reporter: The bucks county district attorney plans to hold a briefing tomorrow to try and answer questions and bring closure to a community who has gone from searching for their missing men to now remembering them. For "Nightline" I'm Eva pilgrim in bucks county, Pennsylvania.
This transcript has been automatically generated and may not be 100% accurate.