ALSEA, Ore. -- A plane crash that killed two friends last year was most likely caused by pilot error, according to National Transportation Safety Board investigators.

The Sept. 12 crash near Alsea killed Duane Jasper Miller, 24, of Billings, Mont., and Kyle Donovan Houghton, 20, of Lake Oswego.

Evidence collected at the crash site showed that the plane's left wing dipped low and likely struck hilly terrain. An examination of the airframe and engine did not find any evidence of structural or mechanical failure, according to the NTSB crash investigation report, which was obtained by the (Corvallis) Gazette-Times newspaper.

Miller and Houghton were raised in Alsea and had friends and relatives there. Miller was a certified flight instructor who often visited from Montana. Houghton was a flight student.

Benton County authorities initially reported that Miller was the pilot and Houghton the passenger. But investigators said the 1968 Piper Cherokee was equipped with dual flight controls, so either man could have been in control when the crash occurred.

Whenever Miller returned home to Alsea, he would fly over the houses of his father and his grandparents. Miller's relatives told investigators they heard a low-flying airplane at 6:45 p.m. Sept. 12 and assumed it was his flight. They also heard what sounded like a crash.

 Background: Plane last seen flying over family home

Rather than immediately contact authorities, the relatives searched for the plane well into the night. Though they didn't find any evidence of a crash, they contacted the authorities the next day because neither Miller nor Houghton had checked in.

The relatives resumed their search on all-terrain vehicles and spotted the wreckage a few hours after contacting the Benton County Sheriff's Office.