He had recently damaged his own garage with his car, police said Thursday.
Police said Russell Weller told them he may have hit the gas pedal instead of the brake Wednesday, hurling victims into the air as his car careened down a street closed to traffic.
"Bodies came up on the car, hit the windshield, went over the top," an eyewitness told CBS News Correspondent Steve Futterman. "Other bodies were being knocked to the side, and he kept going."
"Mr. Weller and his family want to express their deepest sympathies to the victims and their families of the tragic accident earlier today," Weller family attorney Jim Bianco said in a statement Wednesday. "This was an unintentional and unfortunate accident."
Eight victims were pronounced dead at the scene and a ninth, a 50-year-old man, died later at a hospital, spokeswoman Barbara Bishop said. Fourteen others were critically injured.
Market vendor Collin Kidwell tried to give CPR to the 3-year-old girl who was killed.
"I looked at her eyes. Her eyes were just glazed. I knew she wasn't there," he told CBS News Early Show co-anchor Julie Chen.
Weller was not arrested, but authorities were considering whether the case was "manslaughter of some type" and investigating whether he was qualified to drive, said Police Chief James T. Butts Jr.
"There may be some negligence as to his capacity to drive safely," Butts said, although he added that Weller, of Santa Monica, has a valid driver's license.
"We searched the residence looking for things like medications, prescriptions, something that would explain his diminished capacity, if there was any," Butts said on CBS News' The Early Show. "The investigators found evidence that he had struck the back of his garage at least twice. We don't know how recent that was."
"Unless there's intent later proven, I think what we're going to find is we have an 86-year-old driver that may not have been as competent as he needed to be to drive," Butts said in another interview.
The former California state legislator who proposed driving tests for senior citizens, Tom Hayden, said he has learned that Weller had renewed his license without taking a road test when he was 84. He declined to say how he obtained the information.
Weller told police he was leaving a nearby post office and didn't realize until too late that the street was closed for the market, Butts said.
Weller was taken to a hospital for a blood test, and initial results found no traces of alcohol or psychoactive drugs such as antidepressants and hallucinogens.
Mary Roney, who has lived two doors down from Weller and his wife for 30 years, said he has never had any trouble driving and she did not know of any health problems.
"A more careful, gentle, loving person you'll never find," said Roney, who described him as active in the community, including serving on a library board and tutoring students at Santa Monica High School.
Butts said he did not have an estimate of the car's speed. Witnesses said Weller's 1992 Buick was moving very fast down the three downtown blocks of Arizona Avenue that were closed for the street market.
"Sixty miles per hour and it wasn't slowing down. It was flying. And then people down, dead and everything," said David Lang, manager of a shoe store along the market route.
"I heard a car just hit, bang, bang, bang," said Mojgan Pour, 38. "I heard people screaming. By the time I looked, I never even saw the car. I tried to help a man and he died while I was helping him."
Even those who deal with tragedy on a regular basis, like Santa Monica police chief James T. Butts Jr., could not believe what they saw.
"This is the single-most horrific scene of tragedy I've ever witnessed in 30 years in law enforcement," he said.
Weller left police headquarters by late afternoon. Walking unsteadily with a cane, he hugged and smiled at people who picked him up from the police station. He declined to comment to a reporter.
The market, which takes place Wednesdays and Saturdays, bisects oceanside Santa Monica's popular Third Street pedestrian promenade and is near the famous Santa Monica Pier. It draws thousands of shoppers and was slated to close just as the incident occurred at 2 p.m.
A witness, Bahram Manahedgi, 50, said one person was on the hood of the car when it finally came to rest after the incident. A woman's body was crushed beneath the vehicle.
Manahedgi said that when he went to pull the driver out, "His eyes were open and he was alive. I said, 'Do you know what the hell you did?' He said, 'No.' I just opened the door, I pulled him out."
A crowd gathered around the car and "wanted to beat him up," Manahedgi said. "I said, 'He's an old man, leave him alone.'"