Nine people have been killed, including a three-year-old girl, after a car careered into a busy Los Angeles marketplace.
The driver was not under the influence of alcohol or drugs
A further 44 people were injured - 15 of them critically.
The Buick sedan veered off the road and into crowds at the popular Santa Monica farmers' market at about 1345 local time (2145GMT).
Eyewitnesses said people were thrown into the air after being hit by the vehicle, which reports suggest was travelling at up to 70 mph (112 km/h).
One witness said: "I saw people being hit, knocked out, it was the worst thing I've seen in my life."
The driver has been identified as 86-year-old Russell Weller.
Santa Monica's police chief, James Butts, said the driver 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.
Mr Butts said the driver told investigators he tried to stop but may have hit the accelerator instead of the brake.
The driver was not arrested, but investigators were considering whether the case was "manslaughter of some type" and whether he was qualified to drive.
He was allowed to leave police headquarters late on Wednesday.
Walking unsteadily with a cane, he hugged and smiled at people who picked him up from the police station.
Art Aceredo, a California Highway Patrol spokesman, said his department would now compile a profile of the driver.
"We will go back, retrace his steps for the last 72 hours and, depending where that takes us, even further to see what condition the gentlemen was in mentally and to see if there was anything, other than mechanical failure.
"I guarantee that when this is finished we'll know exactly what happened and why it happened, because we owe that to the victims and their families."
Santa Monica's farmers' market is immensely popular, with about 9,000 people attending every Wednesday.