Updated

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.

A statement from California Gov. Jerry Brown received early Wednesday identified BART Sgt. Tom Smith, 42, of San Ramon, as the officer shot Tuesday afternoon. Smith, a detective who's been with the department for 20 years, is survived by his wife and 6-year-old daughter, Brown said.

"Sergeant Smith's family, friends and colleagues are in our thoughts as we honor his service during this painful time," the statement said.

The officers — members of BART's detective unit — were conducting a probation search at an apartment in the East Bay city of Dublin. The governor said the officers were serving a warrant regarding several robberies on BART property.

The suspect, whose name has not been released, was not home at the time.

Sgt. J.D. Nelson of the Alameda County Sheriff's Office, which will investigate the shooting, told the Contra Costa Times (http://bit.ly/1bhi7o6 ) 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.

Smith 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.

BART Police Chief Kenton 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 other details for now. The name of the officer who fired hasn't been released.

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.