LEWISTON — Engine failure may have been to blame for the fatal crash of a small plane 100 yards offshore from Portland Head Light last summer, federal investigators say.

Tourists watched from the shore in Cape Elizabeth as a helicopter dropped a flotation device to Dr. Louis Hanson on June 24 after his four-seat Stinson S108 splashed down off Fort Williams Park, home to the historic lighthouse. Hanson was unresponsive by the time he was pulled onto a boat.

The National Transportation Safety Board found that the bottom portion of one of the six pistons had fractured, causing metal pieces to enter the engine’s camshaft, affecting the magneto timing, the Sun Journal reported Tuesday.

The 66-year-old airplane, based at Twitchell’s Airport in Turner, was running with the original engine, and a mechanic had balanced the pistons in 2007.

Hanson told air traffic controllers before crashing that he had experienced a total power loss at 500 feet and would not be able to glide back to shore, according to the report, which did not reach any conclusions on the cause of the crash.

The 1946 plane, which Hanson bought in 2002, was commonly called a Stinson Voyager.