NEWRY – Pilot error was the likely cause of an accident that killed a pilot and his three passengers when a plane crashed into a rugged mountainside in western Maine during an orientation flight last year.

The National Transportation Safety Board concluded in a probable cause report issued last week that the pilot, William “Charlie” Weir, failed to maintain altitude and clearance before crashing into Barker Mountain on June 22, 2006.

The crash killed Weir, 24, and three Lewiston High School students: Nicholas Babcock, 17, Teisha Loesberg, 16, and Shannon Fortier, 15. The students were participants in a summer camp program run by the school’s Air Force Junior Reserve Officer Training Corps.

The report puts an end to a yearlong investigation. The NTSB earlier this summer released findings saying that Weir was known for flying erratically in training flights carrying Lewiston High School Air Force ROTC cadets.

Weir was working for Twin Cities Air Service in Auburn and providing lessons to the cadets when he crashed.

According to the NTSB report, another teenager who had flown with Weir earlier in the day and on previous occasions told investigators the pilot had a history of showing off behind the controls of an airplane.

Other cadets who flew with Weir earlier on the day of the crash told federal investigators that the pilot flew his plane barefoot as he buzzed them over trails and logging roads before circling the Sunday River Ski Resort, where he encountered difficulties.

Interviews with surviving cadets also revealed to investigators that Weir had encountered flight problems earlier in the day, with the plane stalling out and diving toward the ground before recovering 75 to 100 feet above the treetops and 300 feet from the side of the mountain.

According to the report, the 1979 Cessna’s engine was still running and the propeller spinning when it crashed through trees into the mountainside. The report stated that the plane’s cabin was consumed by fire, and the throttle was full open and the mixture control was in the full rich position.