A San Francisco Bay transit officer was accidentally shot and killed by a colleague Tuesday afternoon while conducting a probation search in a suspect's apartment. This marks the first time that a Bay Area Rapid Transit (BART) officer was killed in the line of duty in the transit police 42-year history.

The shooting occurred at around 2 p.m. while BART police officers were at an apartment complex in Dublin, said Sgt. J.D. Nelson of the Alameda County Sheriff's Office according to the San Jose Mercury News.

According to officials, police knocked on the apartment's door twice, but after no one answered they found that the door was unlocked and entered the apartment. Because they were unaware if anyone was inside at the time, the officers followed protocol and entered with their guns drawn, said Nelson.

While inside, Sgt. Tom Smith, Jr., a 42-year-old veteran with more than 20 years of experience in law enforcement, was shot by a male officer who had more than 10 years on the force. Smith later died at a hospital. Details surrounding the shooting were not immediately released.

Nelson mentioned that the officers were wearing bulletproof vests at the time of the shooting, but he did not say where on his body the slain officer was struck.

The officers were there to serve a warrant for a suspect who was wanted in connection with several robberies that took place on BART property, Nelson said. Officers knew that the suspect, whose name was not released, was in custody at the time, he added.

"This is a shocking thing for a BART officer to be killed in the line of duty," said elected BART Director Gail Murray of Walnut Creek. "But like all peace officers, our BART police risk their lives when they go out to work in the stations and cities. We feel very saddened for the family, who would not expect this to happen to their loved ones."

BART police Chief Kenton Rainey and BART General Manager Grace Crunican also released a  statement, saying, "The entire BART organization is deeply saddened by this tragic event and we ask the public to keep the officer's family in its thoughts and prayers," reports USA Today.