Authorities released the identities of two men, one of whom was from Salt Lake City, who died Friday when a mail plane en route from Billings, Mont., to Salt Lake City plummeted into a mountain near Soda Springs, Idaho.

Clarence Preble, Salt Lake City, and pilot Dave Clark, Billings, Mont., were found dead in the wreckage of a twin-engine Aero Commander 680 Saturday afternoon, said a spokeswoman for Corporate Air Service Inc., owner of the plane.National Transportation Safety Board investigators arrived Sunday at the crash site, 12 miles northeast of Soda Springs, Idaho, said Don Grovak, duty officer at FAA's Seattle office.

The plane had just requested permission from Salt Lake air controllers to change altitude when it disappeared from radar screens about 9:20 p.m. Friday, Grovak said.

The pilot lowered the craft's altitude to a "perfectly normal and very safe" 12,500 feet after receiving permission from the FAA's flight center in Salt Lake City immediately before it crashed, he said.

But preliminary investigation of the wreckage did not reveal a cause of the crash, Grovak said.

"It's going to take our investigators a while to come out with a decision on this one because there isn't much left of the plane," Grovak said.

The plane, carrying U.S. mail, crashed into severe terrain, strewing letters over a wide area, a Caribou County (Idaho) Sheriff's official said.

Preble was also a pilot for Corporate Air, which operates an office in Salt Lake City. He and Clark took off from Billings Logan International Airport about 8:03 p.m. Friday.

Preble had moved to Salt Lake City only three weeks ago and was flying home on the cargo flight after attending to business in Billings, Corporate Air spokesman James Schollenberger said.