LAS VEGAS — Authorities in Nevada were trying Tuesday to find a girl believed to be 4 or 5 years old and a man seen performing explicit sexual acts on her in an amateur videotape that detectives were told was found in the desert.

“The problem is the unknowns,” said Nye County sheriff’s Detective David Boruchowitz, who said he hoped widespread national attention would identify the girl and the man.

“We have no idea where he’s from, where she’s from, and no idea who they are or where they are,” Boruchowitz said in a telephone interview from the sheriff’s office in Pahrump, 60 miles west of Las Vegas.

The tape, a little longer than 30 minutes, clearly shows the adult man performing criminal sexual acts that Boruchowitz said included rape, “everything else, and then some.”

A 26-year-old Pahrump man, Darren Tuck, surrendered the tape to Nye County sheriff’s investigators Sept. 8, after another man reported seeing it, Boruchowitz said.

Tuck told detectives he found the videotape in the desert outside Pahrump more than five months ago, although Boruchowitz said the tape didn’t appear to have been exposed to weather or elements.

Boruchowitz said investigators don’t think Tuck made the tape. Tuck was arrested Sept. 19 on charges of promoting child pornography and possession of child pornography, felonies. The top charge carries a possible sentence of up to life in prison.

Tuck was released without bail, pending arraignment Nov. 26 in Pahrump Justice Court, a court official said.

His lawyer did not immediately respond Tuesday to messages seeking comment.

“Our concern is if little girl is still in that situation,” Boruchowitz said. “Our concern is to get her out of the nightmare that she’s been living in.”