A twin-engine plane slid nose-first onto the eastern shore of Utah Lake on Saturday afternoon, but investigators have yet to figure out why.

The Piper Apache piloted by John Scott Riffle, 21, of Salt Lake City, skimmed the surface of the water before coming to rest nose first on the ground about a quarter of a mile north of the Utah Lake State Park boat harbor. Riffle and his 24-year-old passenger, Alfred Charles Schwab, also of Salt Lake City, walked away from the slightly damaged aircraft uninjured.The state park is about a mile north of the Provo Municipal Airport.

Susan Dillon, an investigator with the Federal Aviation Administration, examined the plane and talked with the pilot but said the reason the craft went down has not been determined. She characterized the situation as an "incident" and not an accident because the airplane received only minor damage and there were no injuries.

She said it would likely take weeks to determine the cause of the unexpected landing.

Sheriff's Lt. Dick Casto said investigators didn't know if the pilot was making an emergency or forced landing.

The pilot would not comment on the incident and referred all inquiries to an attorney, who was not present. "We want to cooperate, but he told us not to really talk to anybody."

The two men left Salt Lake Airport No. 2 about noon in a plane rented from Classic Aviation en route to Las Vegas, Casto said. Utah County police dispatchers received a call on a plane down at 1:42 p.m., he said.

Investigators initially called the unexpected landing suspicious. The sheriff's office called in narcotics investigators and a search dog. The dog sniffed the airplane and five pieces of luggage on board.

Casto said officers found nothing suspicious on the plane or in the luggage. The aircraft was released to its owner for removal, he said.

Also called to the scene was an officer who tests people for drug and alcohol use. After the evaluation, Casto said, "He (the pilot) was not on any drugs or alcohol that we're aware of."

A Salt Lake County man, who asked that his name not be used, said he flew the aircaft earlier Saturday morning. The man, who works near the Provo airport, said he heard through radio transmissions that the plane had gone down and came to the scene to see what had happened.