According to officials, the driver of that double decker bus was driving erratically and speeding for at least two blocks before ending at the site.
Police say he hit a bicyclist, who is in critical condition, before hitting several vehicles and scaffolding which then fell on more cars. That scaffolding is part of construction at the site of the new Apple store.
PHOTOS: Several injuries reported after tour bus accident in SF
"According to eyewitnesses, they saw the bus, which is double decker open air bus, driving erratically down Post Street," said San Francisco Fire Department spokesperson Mindy Talmadge. "In the process, a few vehicles were struck then the bus came to rest against some scaffolding which fell into the street."
First responders had to pull people out because they were pinned beneath the bus and scaffolding.
"PD will investigate what those circumstances involved, whether it was mechanical failure, whether it was driver error," said San Francisco Fire Department Chief Joanne Hayes-White.
Witness John Zimmer added, "It was like a bomb going off. It just kept going boom, boom, boom, boom, boom. Like it wasn't gonna stop."
Police will investigate what happened to the driver to see if it was a mechanical problem or if he suffered a health issue.
Officials say some victims were treated at the scene but others, including the driver, were taken to the hospital.
Stay with ABC7 News for the latest details on this developing story.
SF Fire Dept. says driver of bus was driving erratically for at least one block before hitting scaffolding. pic.twitter.com/RMGgMmunWC— Lyanne Melendez (@LyanneMelendez) November 14, 2015