BART police responded around 8 a.m. to a train operator's report of a man with a gun in his waistband at the station.
Officers arrived on the platform with their guns drawn, giving some morning commuters a scare and causing three trains to be held as police investigated.
After speaking with the man with the concealed gun, police quickly learned that he was an off-duty police officer, BART police Lt. Aaron Ledford said.
"Once we determined that he was off-duty law enforcement, we released the trains," he said.
The incident caused delays of up to 20 minutes, he said.
Although it turned out there was no hazard to the public, Ledford credited the train operator who reported the weapon for his quick reaction in a potentially dangerous situation.