A seemingly routine mission by BART police to gather evidence at a robbery suspect's apartment in Dublin went horribly wrong Tuesday when one of the officers who forced entry into the home accidentally shot and killed another officer, authorities said.

The death of Sgt. Tom Smith, a 42-year-old agency veteran whose wife is also a BART officer, sent grief through the Bay Area's law enforcement community.

But how such a tragedy could happen during a probation search - one involving the empty apartment of a suspect who has been in custody since last week - was not immediately clear.

The shooting was the first on-duty death in the history of the BART Police Department. It happened about 1:45 p.m. at the normally quiet Park Sierra Apartments at 6450 Dougherty Road. Smith was taken to Eden Medical Center in Castro Valley, where he was pronounced dead.

Sgt. J.D. Nelson of the Alameda County Sheriff's Office, which has jurisdiction in Dublin, confirmed that the officer had been shot by a BART colleague. "It was either accidental or target misidentification," he said.

Nelson said the officer who shot the victim is a 10-year veteran. Sources familiar with the investigation identified Smith as the victim and Officer Michael Maes as the shooter.

Brothers in law enforcement

Smith was a 20-year veteran of the BART force who has two brothers in law enforcement - one an Alameda County sheriff's deputy, the other a Newark officer. Smith and his wife lived in San Ramon and have a 6-year-old daughter.

"Our condolences go out to the immediate family and friends and the extended BART family and the entire BART district," said the agency's police chief, Kenton Rainey. "We ask that everyone please give us a chance to catch our breath and grieve."

The shooting comes as BART continues to try to move past a former officer's fatal shooting of unarmed train rider Oscar Grant on Jan. 1, 2009 - a shooting the officer called accidental.

Last month, an independent auditor praised the department's progress in overhauling a force that had been hamstrung by outdated policies, lax oversight and poor officer training.

Nelson said the shooting happened after several BART officers went to an apartment in a modern complex of three-story buildings to search a home connected to a man suspected of committing robberies on BART property.

Sources identified that suspect as John Henry Lee, 20. He was bitten by a police dog and arrested by San Leandro police Thursday after officials said he led officers on a 20-minute chase into Oakland. Lee had been armed with a handgun and was driving a Honda stolen from a BART lot, police said.

The officers who were conducting the search in Dublin - all of whom wore bulletproof vests but were not part of a SWAT team - forced entry into the apartment, Nelson said, before "one officer fired a shot, which fatally struck another officer."

The shooting shocked residents of the nearby apartments, which sit on both sides of the Iron Horse Trail, saying the complex is usually quiet.

"The most I've ever heard is people arguing," said Rosalia Vazquez, a 33-year-old saleswoman who moved in almost a year ago. "This is just crazy. Nothing like this has ever happened here before."

'An amazing man'

Rainey and BART General Manager Grace Crunican released a joint 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."

They said they had visited the officer's family to provide condolences and offer support.

Smith's neighbors in a quiet San Ramon neighborhood off Bollinger Canyon Road described him as a friendly, down-to-earth and devoted father who was often outside playing with his daughter or walking his police dog.

"He was an amazing dad, an amazing neighbor, an amazing man," said Neha Shah, who has lived across the street from Smith's family for four years. "I can't say enough about him. This is heartbreaking."

Shah's husband, Amit Shah, said he heard about the shooting on the radio but couldn't believe Smith was involved.

"This is just surreal. I just saw him this morning," he said. "He and his wife are both such wonderful people. This shouldn't happen."

The investigation is being handled by the Dublin police force, a division of the Sheriff's Office. All of the officers who were at the apartment were later sequestered at the Dublin police station, Nelson said.

Officer's 2001 killing

It is not the first time an officer has been the victim in an accidental police shooting in the East Bay. In 2001, two rookie Oakland police officers shot and killed a plainclothes officer whom they mistook for an armed suspect holding someone at gunpoint.

The mood was quiet and somber Tuesday afternoon at Eden Medical Center, as officers from throughout the region gathered to mourn Smith. There were hugs and tears as they congregated on the hospital's bottom floor.

Outside, officers stood at attention as Smith's flag-draped coffin was loaded into an Alameda County coroner's van. A police motorcycle escort accompanied the van as it drove to the coroner's office in Oakland.