SAN FRANCISCO -- Four people were left in critical condition after a double-decker tour bus in San Francisco went out of control on Friday. The bus hit nearly everything in its path for at least two city blocks.
The open-top double-decker bus ran down a bicyclist, struck two pedestrians and smashed into moving cars and kept going.
"The bus was going very quickly through the intersection -- like at least 40 miles an hour and it looked like it was going out of control," said witness Katy Bailey.
The bus came to a halt at a construction site in the middle of Union Square -- a busy shopping and hotel district in San Francisco.
Security worker John Zimmer called the scene horrific.
"It was like a bomb going off -- just the boom, boom...like it wasn't going to stop," he said.
Thirty people were on the bus when it crashed, 20 were hurt. And eight people, including a minor, were taken to the hospital. Four people remain in critical condition after the crash.
"It looked like there was probably no brakes because it was hitting everything," said witness Jerry Murray.
Investigators have not said if the cause of the crash was mechanical or driver error.
Officer Albie Esparza said the investigation is ongoing.
"We look at any mechanical malfunctions," he said. "We will look at any DUI or alcohol or narcotics as standard operating procedures."
The CEO for the City Sightseeing San Francisco released a statement saying he was "deeply saddened" and is "cooperating with the authorities."
Firefighters said the bus driver was awake and talking at the time of the crash, but his condition is unknown.