A San Francisco driver whose car sped off Bohemian Highway and killed a woman sitting in a nearby parking lot caused the crash by hitting her gas pedal instead of her brake, the CHP said Wednesday.

"It was not a a mechanical failure, she had pedal confusion," said CHP Sgt. Allan Capurro. "She thought she was stepping on her brake but she was stepping on the gas."

The case against driver Johnella Thomas, 74, will go to the District Attorney's Office this week, Capurro said.

Prosecutors will determine if she should be arrested on suspicion of misdemeanor manslaughter, he said.

Nola Mary Wolf, 61, was sitting in the path of the 2010 Mercedes and was killed in the Feb. 24 crash at a Camp Meeker conference center.

Thomas, who was seriously injured, told CHP investigators that her brakes had failed and the car suddenly accelerated.

The investigation into the cause took weeks, in part because of two evaluations of the Mercedes C300 sedan.

CHP investigators checked the vehicle and determined no mechanical malfunction had occurred.

Mercedes Benz company officials also wanted to review the car. A subsequent evaluation was done of the car's computerized electrical system and determined there had been no malfunction.

Investigators also determined that Thomas had not suffered a medical emergency.

She had been driving on Bohemian Highway that morning with a friend when her car began to speed up. A witness driving behind her reported the Mercedes was traveling at a normal speed when it began to accelerate to about 70 mph on the winding highway just prior to the crash.

The car flew off of the two-lane highway and down a short embankment near the entrance to Alliance Redwoods Conference Grounds where Wolf was sitting under a gazebo on the parking lot below. She was with her husband and others, who were waiting for a zipline tree canopy tour.

The car hit Wolf, continued across a parking lot and flew across a creek before smashing into an embankment on the other side.

Wolf was the only person hit by the car. Thomas and her passenger Lillie Richardson, 76, of Marin City, suffered major injuries and were hospitalized.

You can reach Staff Writer Randi Rossmann at 521-5412 or randi.rossmann @pressdemocrat.com.