Authorities are investigating the circumstances behind a fiery helicopter crash at the Sunol Grade that killed a flight instructor and student, knocked out power lines and caused a grass fire early Friday.

The Alameda County Coroner's Office has not released the names of the two male victims, pending identification by dental records.

The men were flying a Schweizer 269 aircraft that clipped power lines and crash-landed onto Sheridan Road near Interstate 680 between Fremont and Sunol around 12:30 a.m., said Ian Gregor, a spokesman with the Federal Aviation Administration.

Fremont Fire Deputy Chief Larry Anderson had just gotten off work and was driving home on I-680 South when he witnessed the crash.

He said the helicopter was flying "very low and very slow," so he was able to keep speed with it in his fire department SUV. At one point, with the helicopter right above him, Anderson rolled down his window and listened to the aircraft's engine. It didn't sound unusual, he said.

Visibility was low as the helicopter flew over the hillside and struck 60,000-volt transmission lines that began to arc. The sky lit up with white flames, Anderson said. He approached the wreckage in hopes of saving the crew, but the fire was too intense to penetrate.