It was reported around 4:30 a.m.
"We're trying to piece everything together and kind of figure out what happened," Sergeant Robert Powers with the CHP said.
VIDEO: Traffic backed up for miles after crash on Bay Bridge
VIDEO: Traffic backed up for miles after fatal crash on Bay Bridge
According to investigators, a box truck carrying three men inside rear-ended a Golden Gate Transit bus carrying 20 people. Two of those people were also injured.
Forty gallons of diesel spilled onto the roadway.
The victim, who was inside the box truck, died. His girlfriend has identified him but ABC7 News is awaiting official confirmation from the coroner's office.
During the initial response to the crash, the other box truck passengers could not give a statement to police.
"Due to their severe injuries, I think the last thing they wanted to talk about was how it happened," Sergeant Powers said. "More focused on what we could do to help them and comfort them," Powers said.
As for the Golden Gate Transit bus - The reason is was on the Bay Bridge is because BART recently stopped running service between 4 and 5 a.m. for construction. So the bus had picked up 20 passengers from the MacArthur BART Station in Oakland to take them to the Sales Force transit center in San Francisco. Two passengers complained of pain and were taken to a medical facility after the accident. Another bus came by to transport the remaining passengers.
The CHP shut down three westbound lanes on the Bay Bridge for several hours. It left most drivers at a standstill.
During the backup, an ABC7 News crew observed someone watching television in their car and a person pulled over to use the restroom in the bushes near the highway.
Officers said motorcyclists are not allowed to ride on the shoulder.
"We don't want any more of these accidents happening as a result of people's impatience," Sergeant Powers said.
Investigators advise commuters to seek alternate routes like BART or the ferry during severe backups.