Deputy Ryan Clinkunbroomer was shot as he sat in his police vehicle at a traffic light in Palmsdale north of Los Angeles.

Authorities in the United States have offered a $250,000 reward for information leading to the arrest of the killer of a deputy sheriff in the western state of California.

Deputy Ryan Clinkunbroomer was shot early on Saturday evening as he sat in his patrol car at a red traffic light in Palmdale, about 90km (55 miles) northeast of Los Angeles.

Sheriff Robert Luna called the shooting a “targeted attack” because the 30-year-old was a law enforcement officer.

“Without warning, he was murdered while serving our community,” Luna told a news conference on Sunday.

Police released grainy surveillance video of a dark saloon car that pulled up alongside Clinkunbroomer’s vehicle moments before the shooting. Luna said investigators believe the “vehicle of interest” is a grey Toyota Corolla manufactured between 2006 and 2012.

Los Angeles County, the city of Palmdale and the Association for Los Angeles Deputy Sheriffs put up a reward of $250,000 for information leading to the killer’s arrest.

“Whoever did this – I’d give in,” Luna said. “We are going to find you.”

California Governor Gavin Newsom called the killing “horrific, unconscionable, and shocking.” He ordered flags at the state capitol flown at half mast in Clinkunbroomer’s honour.

Clinkunbroomer, who had been in the force for eight years and was engaged to be married, was found at the wheel of his car by a passer-by who called for help. He died in hospital.

Luna said the Los Angeles County Sheriff’s office would use every resource, and that local, state and federal officials had offered assistance.

There have been 83 ambush-style attacks on police in the US this year, resulting in 15 killed by gunfire, according to the National Fraternal Order of Police.