Officials identified 20-year BART veteran Sgt. Tom Smith, 42, of San Ramon, Calif., as the victim in the shooting, which took place when Smith and a fellow officer were conducting a search at the East Bay apartment of a robbery suspect who was in custody.
His death is the first on-duty fatality in BART police's 42-year history.
"That's going to be part of the investigation, to find out if this was some sort of an accidental discharge or whether it was a case of target misidentification," Alameda County Sheriff's Sgt. J.D. Nelson said. "Either way it was an accident, and we're going to try to figure out exactly what happened and the circumstances that led up to it."
Smith, along will fellow officers from BART's detective unit, were searching the apartment of an unnamed suspect who is accused of committing robberies on BART property.
The officers entered the apartment with their weapons drawn, as is protocol, because knocks to the door were unanswered and the door was unlocked.
Officials have declined to release details about what led Smith's colleague, who was described as a 10-year veteran of BART, to fire a single shot.
"You also have to understand how devastated he is at this point," Nelson said. "This was certainly not his intent, and you can only imagine the heartbreak that that officer has."
Although the officers were wearing bulletproof vests while conducting their search, Nelson said Smith was hit, and then taken to Eden Medical Center in Castro Valley, where he later died.
Smith's fellow officers stood by at the hospital, standing and saluting as his flag-draped body was wheeled out of the hospital and into a coroner's van, then and a motorcycle escort led the van as it drove away.
Smith is survived by his wife and 6-year-old daughter.
2014: The Year in Music [PHOTOS]