A 61-year-old Petaluma woman was killed Friday when a speeding car ran off Bohemian Highway north of Camp Meeker, plowed into a gazebo where she and a half-dozen people were standing and launched over a creek at a conference center.

The driver and her passenger were hospitalized, but no others were injured in the freak crash, at 1:25 p.m. at the Alliance Redwood Conference Grounds.

"It could have been a lot worse," said CHP Sgt. Bob Barr.

The woman killed wasn't identified pending notification of her family. She was with her husband and at least one friend waiting to go on a popular zipline "canopy tour" through the redwoods, authorities said.

It was unclear why the driver, Johnella Thomas, 74, of San Francisco, suddenly veered off northbound Bohemian Highway, a curvy road where signs warn "slippery when wet" and potentially icy conditions. Conditions were clear and dry Friday.

Barr said a driver behind Thomas about a half-mile before the wreck saw her 2010 Mercedes C300 traveling at 25 mph to 30 mph, near the posted speed limit.

The witness said Thomas suddenly accelerated and pulled away, going so fast she could barely stay on the road.

He estimated the Mercedes' speed at 70 mph.

"He lost sight of the car, then came around the corner and saw dust and carnage," Barr said.

The victim was standing near an open redwood platform covered by a pop-up gazebo, where customers of the canopy tours gather before being escorted to another section of the conference center to visit the redwoods, said Jim Blake, the center's executive director.

Mary Krieger, 63, of Indiana and Ginger Bird, 46, of Vacaville were among those standing nearby; the others weren't identified.

They were not seriously injured, although some people in the group may have been struck by flying debris.

Barr said the CHP hadn't ruled out any causes — mechanical, health problems for the driver or drugs or alcohol use, although he said there wasn't any indication of those substances at the scene.

Thomas and her passenger, identified only as an adult woman, were taken by ambulance to Santa Rosa Memorial Hospital, where Thomas was listed in critical condition Friday night. Information about her passenger was not available.

In the minutes after the crash, dozens of customers and employees of the conference center milled around the wooded grounds, stunned looks on their faces as emergency personnel tended to the victims and talked with witnesses.

Law enforcement chaplains offered solace to witnesses.

Many said they were too shocked to comment. Others who didn't want to be quoted simply marveled that only one person was struck.

One man, who didn't give a name, said he heard terrible crashing sounds and turned to see a path of destruction and bedlam near the gazebo for the canopy tours, an attraction that opened in 2010.

Splintered redwood-posts lay scattered around a 100-foot swath of the parking lot. Small pieces of car parts littered the Mercedes' path.

After striking the gazebo, the car continued through the parking lot, launching over a shallow creek near a bridge to the center's offices.

The car continued airborne for about 20 yards over the creek, landing on its right side against a large concrete abutment on which a picnic table sits.

The accident created a frenzied scene at the conference grounds, which hosts public school students and other groups in outdoor education programs for daytime and overnight events.

"There's just a lot of chaos right now," said Bruce Wohlert, director of the conference center, shortly after the crash. "We're trying to sort out the details."

In July 2010, the parking lot at the conference grounds was the scene of another fatal accident. The driver of a pickup was killed when the truck left the highway, hit a redwood tree, crashed down an embankment and burst into flames in the parking lot.

Staff Writer Mary Callahan contributed to this story.