Over a two-day period starting last Wednesday night, four young men in Bucks County, a wealthy Philadelphia suburb where new construction meets large estates, disappeared.

The first to vanish, Jimi Tar Patrick, 19, was last seen around 6 p.m. on Wednesday in Newtown Township, Pa., and did not show up for work the next day, the authorities said.

On Friday around 6 p.m., Mark Sturgis, 22, told his father he was going to meet up with his longtime best friend, Tom Meo, 21, and that they would be ready for their construction jobs the next morning. They did not show up, and their cellphones went straight to voice mail.

Dean Finocchiaro, 18, was last seen about 6:30 p.m. on Friday in Middletown Township.

In the hours after the four men disappeared, the police departments and the district attorney’s office in Bucks County — where violent crime is considered rare — cast a wide net, searching throughout the county and requesting the public’s help on social media. On Saturday afternoon, the authorities got a break: Mr. Finocchiaro’s cellphone was tracked to a vast farm off a country road in Solebury Township.

On Monday, the police arrested the son of the farm’s owners, Cosmo DiNardo, 20, on a prior felony weapons charge, according to court records. They also searched the farm with heavy equipment. Mr. DiNardo, who was being held on $1 million bail in the Bucks County Jail, had the charge initially dismissed in May. The authorities declined to say whether they believed Mr. DiNardo played a role in the disappearance of the four men.

Efforts on Monday to reach Mr. DiNardo’s parents or his lawyer by phone were unsuccessful.

The officers who followed the cellphone signal on Saturday did not find the young men, but they did discover another clue. An early 1990s champagne-colored Nissan Maxima, which belonged to Mr. Meo, was there, according to Mark Potash, the father of Mr. Sturgis. The county’s district attorney, Matthew Weintraub, immediately redirected the search team, bringing in all-terrain vehicles and heavy equipment for what was now considered a criminal investigation.

“We’re getting a lot of strong indications that this is where we need to focus our resources,” Mr. Weintraub said at a news conference on Monday afternoon, about a half-mile from the search site. “We are trying to find needles in a haystack.”

While the bulk of the investigation has focused on the Solebury Township farm, the authorities have also searched the home of Mr. DiNardo, who lives in Bensalem, about 30 miles from his parents’ property. The police also recovered Mr. Sturgis’s car, a 2004 Nissan Altima, at an outdoor shopping mall, Mr. Potash said.

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Mr. Weintraub declined to say on Monday whether the men were believed to be alive. “We are not sure what we are going to find there, but we are pretty confident that the investigation is proceeding in the direction that we believed that it would,” he said.

Throughout the day on Monday, the parents of the missing men gathered near a cluster of police cars on the property, as investigators combed the farm for any signs of the men, Mr. Potash said. As helicopters circled overhead, the police used a backhoe to dig up part of the ground, including some concrete.

After nearly a day’s work, the police told the families that there were no signs of the men, but that they were confident they were searching in the right area, Mr. Potash said. He added that the authorities were doing a “tremendous job” of helping the families search for their children.

“It’s so horrible,” he said. “I have never experienced anything so devastating in my life. I had to come home to try to get my head together.”

Mr. Potash said that his son and Mr. Meo, who both work for his construction company, met a decade ago while attending school in Bensalem. He said he did not believe they knew Mr. Patrick, but added that he had overheard his son and Mr. Meo mention Mr. Finocchiaro by name.

It is also unclear whether Mr. DiNardo has any connections to them. On Facebook, Mr. DiNardo is friends with Mr. Patrick, who disappeared last Wednesday.

Mr. DiNardo was first arrested on a felony weapons charge in February because his history of mental illness prohibits him from possessing a weapon, The Associated Press reported, citing police records.

Rich Turchi, a longtime friend, said that Mr. DiNardo worked in a concrete construction business owned by his father, and was a hunter. “It’s outrageous,” Mr. Turchi said of his friend’s arrest on Monday. “It’s all fishy.”

At the news conference on Monday, Mr. Weintraub said he would continue to devote significant resources to the case and stay at the farm until the search was complete. “We are not going to rest until we get through every inch of that property,” he said.