Authorities in Idaho are trying to determine what caused an SUV to go off a steep cliff and into a reservoir, killing a woman and her three children.

Investigators say they'll study the wreckage to determine if the vehicle had any mechanical problems.

The Ada County Sheriff's Office says an autopsy also will be done Friday on the body of the mother, a 40-year-old from Boise.

The crash also killed her 6-year-old and 12-year-old daughters and her 10-year-old son. The Ada County Coroner's Office is expected to release their names in the next few days.

The SUV plunged about 70 feet off a cliff and into Lucky Peak Reservoir on Thursday.

Sheriff's officials say a witness saw the vehicle first accelerate from the side of the road. Deputies say they found no skid or brake marks where the vehicle went over the edge.