San Francisco Police believe the tour bus that crashed injuring 20 people on Friday was likely caused by a mechanical problem.
Four people were in a critical condition after the double decker bus ploughed into cars after striking a cyclist in the busy tourist district of Union Square.
"It appears to be a mechanical defect with the vehicle itself," said San Francisco supervisor Jane Kim.
"I think there are questions about how the record keeping has been kept, and also how vehicles are being maintained by many of these very large tour bus companies regulated by the state," Kim said.
The open-top bus was operated by tour firm City Sightseeing San Francisco.
A statement from its CEO Christian Watts said: "We are cooperating with the authorities to determine how this happened. We pray that everyone involved recovers fully and quickly."
The bus also crashed into scaffolding bringing down debris which trapped some of the occupants inside the bus.
According to a NBC San Francisco news report, the bus was seen driving erratically two blocks before coming to a sudden halt.
At least 30 passengers were onboard the bus at the time.
Several cars were wrecked in the collision and at last two pedestrians were trapped under the bus, emergency services said.
