An initial investigation into a deadly helicopter crash that killed nine people in Northern California earlier this month has found that the chopper's main rotor lost power during takeoff.

A preliminary crash report released by the National Transportation Safety Board says the Sikorsky S-61N helicopter hit trees after losing power and fell out of the air over the Shasta-Trinity National Forest.

The report says the firefighting aircraft came to rest on its left side before bursting into flames. Four of the 13 people on board survived with various injuries.

The helicopter was ferrying firefighters away from a remote site on the front lines of a stubborn wildfire when it crashed the evening of Aug. 5.