This story has been updated to include a statement from BART Police Chief Kenton Rainey.

SAN FRANCISCO — Authorities looking into how a Bay Area Rapid Transit District (BART) officer was fatally shot by his partner Tuesday told the Los Angeles Times that it is not clear whether the weapon fired accidentally or the shooting was due to a "misidentified target."

The incident occurred while members of BART's detective unit were conducting a probation search on a suspect at an apartment complex, Alameda County Sheriff's Sgt. J.D. Nelson, whose department is investigating the incident, told the paper.

The officer was the first in the department's history to die on duty, a BART police chief told reporters. For the full story, click here.

Additionally, the transit system released on Tuesday a statement from BART Police Chief Kenton Rainey:

"On behalf of the SF BART, I am here this morning to confirm the reports that it was in fact Sgt. Tommy Smith who was tragically killed in the line of duty yesterday afternoon. As it was reported last night Tommy and other members from the department were conducting a probation search in the City of Dublin when he was fatally struck by gunfire from a fellow officer. Paramedics were summoned to the scene and immediately transported him to Eden Medical Center in Castro Valley. However, despite the emergency room medical staff's best efforts they were unable to save Tommy.

Tommy was a 23-year veteran with the department and was in charge of our detective unit. He was married, and he and his wife Kellie, who is a BPD K-9 officer, have one daughter. Tommy was a great law enforcement officer, but an even better son, brother, husband, father and friend. He touched many lives in a positive way both professionally and personally and he will be sorely missed. On behalf of the BART Board of Directors, the GM and the rest of the District, I would like to extend our deepest condolences to Kellie and their daughter, and the other members of their immediate family.

This tragic incident is being investigated by the following agencies: Alameda County Sheriff’s Office, Alameda County DA’s Office, BPD Internal Affairs Unit and BART Independent Police Auditor."