PHILADELPHIA >> Police used cadaver dogs, a backhoe and other construction equipment today to help search a sprawling farm for four missing men believed to be victims of foul play. A prosecutor, meanwhile, described a man who was held on an unrelated gun charge, but has since been released on bail, as a person of interest in the investigation.

District Attorney Matthew Weintraub said bail was set at $1 million Monday for Cosmo DiNardo, whose family owns the farmland and another property that was searched, because he was considered a flight risk. But Weintraub did not call him a suspect and cautioned there is often a “chasm” between being called a “person of interest” in an investigation and being charged with a crime.

DiNardo, 20, is accused of illegally possessing a shotgun and ammunition in February. A court affidavit said he had suffered from mental illness and had been involuntarily committed to an institution for inpatient care. It did not say when the commitment had occurred. The charge had been dismissed by a judge but was refiled.

Late today, DiNardo was released after he posted 10 percent of his bail amount in cash, according to the Bucks County District Attorney’s Twitter account.

A busload of police cadets also took part in the third day of the search of farmland in Solebury Township, about 30 miles north of Philadelphia.

Aerial TV footage today showed law enforcement officers crowded around a deep trench — a backhoe parked next to it — as officers used shovels to dig deeper and passed up buckets of dirt that were then screened with sifters.

“We want to be careful not to miss the tiniest piece of evidence,” Weintraub said at an earlier briefing.

The missing are 22-year-old Mark Sturgis, 21-year-old Tom Meo, 19-year-old Dean Finocchiaro and 19-year-old Jimi Tar Patrick, a student at Loyola University in Baltimore. Patrick disappeared on Wednesday, the other three on Friday.

Sturgis and Meo are longtime friends who work at the Sturgis family’s construction business, and Finocchiaro is a mutual friend, Sturgis’ father has said.

Police have received “a ton of leads” and are making progress, but the entire 90-acre (0.14-square-mile) property is of interest to investigators, Weintraub said. He said investigators were also working across the county.

The farm property is owned by DiNardo’s parents, Antonio and Sandra DiNardo. Neither the DiNardos nor their son’s lawyer could be reached for comment.

Peter Dragani, a Bensalem real estate agent who said he coached Cosmo DiNardo in a youth football league and has stayed in touch with the family over the years, described the jailed young man as a “model citizen.”

“He comes from a good family, a strong, very strict family,” he told the Associated Press. “I never met anybody that didn’t like him.”

Asked if the four missing men could still be alive, Weintraub said, “I think it’s very important to hang on to hope.”

The FBI, state police and at least five local law enforcement agencies were also taking part in the search.