CHP officers returned to Camp Meeker on Monday, trying to pinpoint what caused a 74-year-old driver to speed off Bohemian Highway and crash into a nearby gazebo, killing a Petaluma woman.

Investigators have determined the driver, Johnella Thomas, did not suffer an obvious medical emergency that triggered the Friday afternoon crash, CHP Officer Jon Sloat said Monday.

Thomas, 74, of San Francisco, survived the accident but incurred major injuries. She remained at Santa Rosa Memorial Hospital on Monday in serious condition.

Her passenger, Lillie Richardson, 76, of Marin City, also remained at Memorial. She was in critical condition.

Thomas could face a charge of involuntary manslaughter involving the death of Nola Mary Wolf, 61.

Wolf had been seated next to her husband in the gazebo at the Alliance Redwoods Conference Grounds outside Camp Meeker.

A 2010 Mercedes C300 driven by Thomas flew off the road and plowed through the gazebo, striking Wolf. Amazingly, none of the other people waiting in the area for a zipline tour through the redwoods were injured.

A witness driving behind Thomas told officers the car initially was traveling at a reasonable speed on the curving highway. He then saw the car accelerate, perhaps as fast as 70 mph.

Thomas told officers her brakes had failed, Sloat said. But a lack of braking wouldn't have caused the car to speed up, he said.

"It's either mechanical, a health issue or confusion on the part of the driver. Maybe she went to step on the brake and ended up stepping on the gas," Sloat said.

Officers Monday were analyzing the scene and documenting details to help determine the car's speed.

"It's a chaotic scene that has to be diagrammed down to the inch," said Sloat.

They didn't find skid marks, indicating brakes hadn't been used before the car left the road.

An inspection of the car will be conducted this week.

Thomas doesn't have a negative driving history.