A preliminary report by federal investigators says witnesses saw a single-engine airplane spinning out of the sky moments before it crashed last month and killed the two men on board.

The National Transportation Safety Board report on the Aug. 15 crash that killed 19-year-old flight student Justin Robbe and his 61-year-old instructor, Richard Johnson, provides some new details, but doesn't conclude what caused the crash.

The Piper PA-38 Tomahawk took off from Butte's Bert Mooney Airport at 10:30 a.m. The flight lasted for about an hour.

One witness told investigators the plane was in a tight spin that loosened before it fell out of sight below a hill.

The wreckage has been recovered and moved to a secure area for the investigation.

Associated Press