A Bay Area Rapid Transit officer who was accidentally shot by his partner and died Tuesday was the first in the department┬┐s history to be killed in the line of duty.

SAN FRANCISCO -- Authorities investigating how a Bay Area Rapid Transit officer was fatally shot by his partner Tuesday say it is unclear whether the weapon fired accidentally or the shooting was due to a "misidentified target."

A somber BART police chief told reporters Tuesday that the officer was the first in the department's history to die on duty.

The members of BART's detective unit were conducting a probation search at a sprawling apartment complex in the eastern Alameda County bedroom community of Dublin. Alameda County Sheriff's Sgt. J.D. Nelson, whose department is investigating the incident, said the suspect, who was already in custody, was wanted in connection with "some crimes that had occurred on a train."

Photos: BART officer fatally shot by partner

During a search shortly before 2 p.m. at the sprawling Park Sierra Apartments, "it appears one officer accidentally fired his weapon, which struck the other officer and ultimately it was a fatal shot," Nelson said.

Nelson said investigators are still trying to determine whether the weapon fired accidentally or the shooting was the result of a "misidentified target."

BART officials have not identified the slain officer, but a website dedicated to fallen officers has identified him as Tom Smith. The officer had been on the force for about 20 years, Nelson said.

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lee.romney@latimes.com

Twitter: @leeromney