A 20-year-old linked to the property where the FBI is searching for four missing Pennsylvania men was arrested Monday and held on $1 million bail on a gun charge, although authorities did not call him a suspect in the feared foul play.

Cosmo DiNardo, of Bensalem, was accused of possessing a shotgun and ammunition in February despite a history of mental illness that includes an involuntary commitment, according to a police affidavit.

His arrest comes as the FBI uses heavy equipment to search his family's sprawling farm property in Solebury Township, about 30 miles (48 kilometers) north of Philadelphia.

Authorities on Monday chased fast-developing leads in the search for four young men, but said it could take days despite intense efforts centered on a large swath of the family's farmland. Six other police agencies were involved.

"The leads are incredibly hot, they're very fruitful. We're making great progress, but there's so much more work to do," Bucks County District Attorney Matthew Weintraub said. "We're not going to rest until we get through every inch of that property."

The leads gave police "a lot of strong indications" that the search should focus on the farm. However, other spots also were being searched as police probed the ties among the men.

One of the missing men vanished Wednesday and three others on Friday. Asked if foul play was involved, Weintraub said, "I surely believe there is."

Two of the men, 22-year-old Mark Sturgis and 21-year-old Tom Meo, are longtime friends who work in construction for Sturgis' father, Mark Potash. Another man, 19-year-old Dean Finocchiaro, is a mutual friend of theirs, Potash told The Associated Press.

Potash said he did not know where his son was heading or what he was doing before he disappeared Friday. News outlets have reported that cars belonging to both Potash and Meo have been found in the area.

"I don't know how this is going to end up, unfortunately, but I am confident that we are going to get to the bottom of this," Weintraub said.

Neither DiNardo's parents nor his lawyer returned messages seeking comment Monday. Weintraub's spokesman stressed that the gun charge against DiNardo does not involve the missing men. The case was refiled after charges first filed in February were dismissed, according to court papers and the police affidavit obtained from the Bucks County Courier Times.

Weintraub described the investigation as "all hands on deck," and said the FBI, state police and five local police departments had joined the case. The FBI was involved because of its expertise with recovery operations being deployed on the Solebury Township property.

One of the missing men, Finocchiaro, has been arrested multiple times on charges that include drug possession, driving under the influence and assault. He is due in court in the assault case this summer.

"He seemed like he was getting it together. He was working full time," said his lawyer, John Fioravanti Jr. "I feel bad for his parents."