BART (Bay Area Rapid Transit) Sgt. Tom Smith was shot and killed Tuesday by a fellow officer as law enforcement searched a suspect’s Dublin, Calif. home for a laptop and other stolen items, police said Wednesday.

Smith and other officers were on a probation search of a suspect’s home. The occupant was already in custody, and Smith’s partner shot him while officers searched over the premises. The incident occurred at around 1pm.

Sgt. Smith was a 23-year veteran of the BART, which patrols the train system in Northern California.

It is believed that the officer who shot Sgt. Smith either mistook him for someone else or the weapon accidentally fired, Alameda County sheriff’s Sgt. J.D. Nelson said.

In a brief statement, a somber BART police chief said the officer was the first to die on duty in the department’s history. Taking no questions, Chief Kenton Rainey said he was confirming the “tragic loss” with “a heavy heart.”

“Our condolences go out to the immediate family and friends and the extended BART family,” Rainey said. “We ask that everyone please give us a chance to catch our breath [and] grieve.”

Although BART officials have not yet identified the officer who fired the shot, a website dedicated to fallen officers has added him to the website, as Tom Smith.

Sgt. Smith had been on the BART force for more than 20 years, Nelson said. The officer who fired his gun had been on the force for a decade.

Both officers were wearing bulletproof vests.

Officer Smith was taken to Eden Medical Center in Castro Valley, where he died.

“BART has been informed that one of our officers has died from wounds sustained during a shooting earlier today,” Rainey and General Manager Grace Crunican said in a joint statement earlier in the evening. “The entire BART organization is deeply saddened by this tragic event,” they said, “and we ask the public to keep the officer’s family in its thoughts and prayers.”

Image via BART Website