Bay Area Rapid Transit police Chief Kenton Rainey, center, speaks to the media outside Eden Medical Center in Castro Valley, Calif., Tuesday, Jan. 21, 2014. Rainey announced the tragic loss of the first BART officer killed in the line of duty. The officer was fatally shot while serving a probation search warrant at a residence in Dublin, Calif. (AP Photo/The Contra Costa Times, Anda Chu)

DUBLIN, Calif. - Investigators are working to determine the circumstances that led a Bay Area Rapid Transit police officer to accidentally shoot and kill a fellow officer, the first on-duty fatality in the department's 42-year history, authorities said.

The officers — members of BART's detective unit — were conducting a search at an apartment in the East Bay city of Dublin when the shooting occurred Tuesday afternoon. BART Police Chief Kenton Rainey said the officers were following up on a crime committed on BART property.

The suspect, whose name has not been released, was not home during the search.

Sgt. J.D. Nelson of the Alameda County Sheriff's Office, which Rainey says will investigate the shooting, told the Contra Costa Times that officers knocked before entering the unlocked apartment with their guns drawn.

Details about what happened next haven't been released, but Nelson told the newspaper that an officer fired one shot.

The wounded officer was taken to Eden Medical Center in Castro Valley, where he died from his injuries.

Television reports showed lines of officers outside the hospital saluting as their fallen comrade's body, draped in a large American flag, was loaded into a coroner's van.

Rainey didn't answer any questions from reporters at a brief news conference Tuesday evening.

"We ask that everyone please give us a chance to catch our breath" and to grieve, he said.

"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," Rainey and BART General Manager Grace Crunican said earlier in a joint statement.

They said they were withholding the officers' identity and other details for now.

BART has had its share of troubles in the past.

Among them was the fatal shooting on New Year's Day 2009 of Oscar Grant III, an unarmed black BART passenger who had been detained at the Fruitvale station after reports of a fight.

Officer Johannes Mehserle, who is white, drew his gun and shot Grant in the back as he lay face down on the platform. The event was recorded by many video and cellphone cameras and was followed by a series of large protests.

Mehserle was convicted of involuntary manslaughter and sentenced to two years minus time served.

An independent auditor said last month that BART police have made significant progress in meeting reforms instituted after Grant's death, including increased officer training about bias and other issues, along with better reporting about incidents involving use of force.

Rainey said BART officers now receive 40 hours of such training every year, compared with the state standard of 24 hours every two years.

The BART Police Department began operation in 1972, but the department traces its beginnings to several years before that.

In 1969, the transit district's board of directors decided that local police and sheriff's departments should patrol the new BART stations and other properties. Police chiefs and sheriffs worried that the proposal would cause disputes and inconsistent levels of police protection, so instead, legislation was passed to create a separate BART Police Department.

News from © The Associated Press, 2014