REDDING, Calif. (AP) — Witnesses to a fiery helicopter accident that killed nine people told investigators the aircraft had lifted off more slowly than normal before it struck a tree and crashed in a remote northern California forest, officials said Friday.

After the chopper's nose hit the tree about 40 to 50 feet above ground, its rotor blades struck trees and branches before the aircraft plummeted to the ground Tuesday night, said National Transportation Safety Board member Kitty Higgins, citing accounts from 10 witnesses.

The aircraft came to rest on its left side about 150 yards from its takeoff site and then "quickly filled with very dense, thick black smoke" before igniting in a fiery blaze, Higgins said at a news conference in Redding, about 40 miles southeast of the crash site.

The Sikorsky S-61N was ferrying 10 firefighters, two pilots and a U.S. Forest Service employee back to base camp when it crashed in the Shasta-Trinity National Forest. Four of those aboard were rescued and taken to hospitals.