An 86-year-old driver who plowed his car into a farmers market in California, killing 10 and injuring 63, mistakenly stepped on the gas rather than the brake, federal safety investigators said Tuesday.

The National Transportation Safety Board (search) ruled the probable cause of the July 16, 2003, accident was "unintended acceleration." The accident was made worse when the driver, George Weller (search), panicked and continued careening through the crowd, investigators said.

The safety board ruled out mechanical failure, weather, alcohol or drugs, and fatigue.

Those who believe older drivers (search) should have to prove their driving ability cited the Santa Monica, Calif., accident as an example of the tragedy that can result when people are too old to drive safely. But the safety board declined to address the issue, instead focusing on the need for permanent barriers at regularly recurring outdoor events like street fairs and farmers markets.

Investigators were unable to interview Weller, who has hired a lawyer as he fights vehicular manslaughter charges and civil lawsuits.

"Had we had access to the driver, we may have ben able to say more about his ability to drive," investigator Joseph Osterman said.

California investigators said Weller had his "eyes open, hands on the steering wheel at the 10 o'clock and 2 o'clock position" and avoided parked cars on both sides as his Buick sedan leveled stands and hurled pedestrians into the air.

He had collided at low speed with a Mercedes-Benz before reaching the market, then drove past a sign saying the road was closed ahead. He then covered 995 feet at speeds up to 60 mph while barreling through the open-air market.

Investigators said Weller was taking prescription drugs that cause nausea and dizziness, and had hip replacements that reduced his mobility.

Weller has pleaded not guilty and is free without bail on the condition he not drive. If convicted on all counts, his sentence could range from probation to 18 years in prison, according to the district attorney's office.

More than 10 civil lawsuits have been filed against Weller. Lawsuits have also been filed against the city of Santa Monica, the company that oversees the market, a farmers association, Los Angeles County's agricultural commissioner and the state of California.

Days after the crash, Weller issued a statement saying he was distraught and heartbroken, and his attorney called it an accident.