PHILADELPHIA – Authorities say they believe foul play may be involved in the disappearance of four young men who went missing within days of each other in Bucks County, Pennsylvania, last week.

Investigators are chasing fast-developing leads in the search for the men, but said Monday that it could take days despite intense efforts centered on a large swath of farmland.

"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 at an afternoon news conference, his second of the day. "We're not going to rest until we get through every inch of that property."

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The men have been identified as Mark Sturgis, 22, of Pennsburg; Tom Meo, 21, of Plumstead; Dean Finocchiaro, 18, of Middletown; and Jimi Tar Patrick, 19, of Newtown Township. Patrick disappeared Wednesday, and the three others on Friday.

The leads gave police "a lot of strong indications" that the search should focus on the cornfield in Solebury Township, about 30 miles north of Philadelphia, but Weintraub would not detail specifically what led investigators there.

'We're not exactly sure what we're going to find," Weintraub said.

However, other spots were also being searched as police investigated the ties among the men. Authorities believe all four know each other. They're calling the case a criminal investigation.

Asked if foul play was involved, Weintraub said, "I sure believe there is."

When asked whether he believed the men were still alive, he said,"I can't say. We hope and pray that they are, but we have to go where the investigation leads us."

Two of the men, Sturgis and Meo, are longtime friends who work in construction for Sturgis' father, Mark Potash. Finocchiaro is a mutual friend of theirs, Potash told The Associated Press.

Sturgis and Meo were last seen together Friday night in the Doylestown area. Finocchiaro has been arrested multiple times on charges that include drug possession, driving under the influence and assault.

Potash said he did not know where his son was heading or what he was doing before he disappeared Friday.

"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.

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 in the farm fields.

If you have any information on the whereabouts of these men, please call 215-297-8201.