Sgt. Tom Smith was wearing a bulletproof vest when he was shot on Tuesday, but the bullet struck an area that was not covered, Alameda County sheriff's spokesman J.D. Nelson said.
"Unfortunately, bulletproof vests do not provide 100 percent support," he said.
Authorities were still trying to determine whether the other officer's weapon discharged accidentally, or if the officer mistook Smith for someone else, Nelson said. Either way, it was an accident, he added.
On Thursday morning, the body of Sgt. Smith was delivered to a funeral home in Hayward. A line of officers saluted in respect and offered their sympathies to his grieving wife, who is also on the force.
Sgt. Smith was a 23-year veteran of the BART force. He was a husband and father.
The BART Board of Directors held a moment of silence in his memory at their meeting Thursday.
BART's police chief says he's met with both the family of the fallen sergeant and the family of the officer who shot and killed him. He says they're doing well. Of course, they were on the minds of everyone at Thursday's BART Board meeting.
After a moment of silence at the meeting, BART Police Chief Kenton Rainey spoke at length about the fallen sergeant. The chief became choked up, telling about how the community has shown so much caring.
"We got a lot of outpouring support from the public and law enforcement around the country," he said.
As for the shooting that took the life of Sgt. Smith, a source tells ABC7 News that the officer who fired the fatal shot is Michael Maes.
The chief says multiple investigations are underway. The Alameda County Sheriff's Department and district attorney's office are among the agencies investigating.
And after being asked by a BART Board member, Rainey acknowledged that two uniformed officers did have video cameras at the time of the shooting. But he said he doesn't yet know what those cameras captured.
Dispatch traffic on the BART police radio frequency reveals a call for an ambulance and then a few seconds later, an ominous indication of what happened.
Dispatch: "Is anybody hurt?"
Officer: "David 34, this was friendly fire."
Dispatch: "Copy, friendly fire."
"Obviously something went wrong and we will not hide from the truth," Rainey said. "Whatever comes out comes out and we will make the necessary to changes to address them."
A viewing for Sgt. Smith will be held Tuesday at Chapel of the Chimes in Hayward from 7 p.m. to 8 p.m. The funeral will be held Wednesday at 10 a.m. at Neighborhood Church in Castro Valley. Both events are open to the public.
The Tommy Smith Memorial fund has been set up at Wells Fargo Bank. Deposits can be made at any Wells Fargo Bank, Account #5148561086 under Kellie Smith.
Or they can be mailed to:
Tommy Smith Memorial Fund C/O Wells Fargo
11020 Bollinger Canyon Road, Suite 1
San Ramon, CA 94582
(The Associated Press contributed to this report)