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An encounter on the road between an Alhambra police officer and a Los Angeles County sheriff’s deputy while both were off-duty in San Marino ended with one of them firing his gun and injuring himself, authorities said Monday.

The incident unfolded around 8:30 a.m. Sunday when the deputy noticed a motorist he said was driving erratically on Duarte Road east of San Gabriel Boulevard in East San Gabriel, the San Marino Police Department said in a statement.

Using his personal vehicle, the deputy followed the driver into the city of San Marino, according to San Marino police.

“The deputy indicated that he wanted to stop the driver and tell him not to speed in the neighborhood,” the agency said.

The deputy then pulled alongside the other vehicle and motioned for the driver to pull down his window, officials said. He did not know that the other motorist was an off-duty Alhambra police officer, according to investigators.

The police officer, also using his personal vehicle, slowed down and moved to the right to let the off-duty deputy pass, authorities said.

The deputy was traveling at high speeds, and this was perceived by the police officer as “aggressive,” according to the San Marino Police Department.

The Alhambra official feared for his safety and drew his firearm while inside his car, and in doing so fired one round that led to his own injury, the Police Department said.

Neither of the officers knew that the other one was also in law enforcement, the Police Department said.

That police officer was taken to a hospital.

Investigators previously described the case as a “road rage incident,” but San Marino police later said that was not the case. The deputy involved is assigned to the L.A. County Sheriff’s Department’s Court Services Division, authorities said.

An earlier police statement indicated that the deputy had been driving a dark-colored Mercedes and that the police officer was traveling in a blue Subaru.

Detectives continue to gather evidence, conduct interviews and search for any witnesses in the case, officials said.

Anyone with information can email Detective Golden at dgolden@smpd.us or call the San Marino Police Department at 626-300-0719. Those who wish to remain anonymous can contact Crime Stoppers by calling 800-222-TIPS (8477), using the mobile app “P3 Tips” or visiting lacrimestoppers.org.