A deputy with the Los Angeles County Sheriff's Department was shot in the head and severely wounded by an unknown assailant at a fast food restaurant on Monday.

According to ABC News , the off-duty deputy—whose name has not yet been released—was transported to a nearby hospital in critical condition. He was still in critical condition Tuesday afternoon.

The gunman escaped the scene of the shooting on foot. A person of interest was being detained Tuesday.

Long Beach, CA, police spotted a vehicle around 10:40 a.m. Tuesday that matched the description of the SUV driven by the assailant, according to the Sheriff’s Department. Officers stopped the vehicle and detained the driver, reports the Los Angeles Times .

"Whomever carried out this cowardly attack; we won't rest until we find you and ensure justice is brought swiftly upon you," Sheriff Alex Villanueva wrote on Twitter late Monday.

Los Angeles County supervisors on Tuesday voted unanimously to establish a $50,000 reward for information leading to the gunman’s arrest. The Assn. for Los Angeles Deputy Sheriffs, the union that represents rank-and-file deputies, also pledged $50,000 of its funds toward a reward for information in the case, said the organization’s president, Det. Ron Hernandez.

Given the circumstances of the attack, detectives are investigating whether the deputy was intentionally targeted by the shooter. He was off duty and wearing plain clothes with no holster, badge or anything to signify he was a law enforcement officer, Capt. Kent Wegener said at a news conference Monday night.