Deputy Ryan Clinkunbroomer, 30, was shot Saturday evening as he sat in his patrol car at a red light just outside a sheriff’s station in the city of Palmdale, about 90km northeast of Los Angeles.

Sheriff Robert Luna called the shooting a “targeted attack” on Clinkunbroomer because he was a law enforcement officer.

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

A security video showed the car – a gray Toyota Corolla – that the shooter used as he carried out the killing, Luna said. But as of Sunday night, no suspects had been identified.

“Please, I beg you, somebody has information, make things right,” he said.

Los Angeles county, the city of Palmdale and the Association for Los Angles deputy sheriffs put up a reward of US$250,000 for information that leads to the arrest of the perpetrator.

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

Clinkunbroomer was found at the wheel of his patrol cruiser gravely wounded about 6pm by a passerby who called for help, police said. Clinkunbroomer, who served for eight years on the force, died at the hospital.

Clinkunbroomer, who was just engaged to be married, was a third-generation member of the Los Angeles County Sheriff’s office, following in the footsteps of his father and grandfather, the sheriff said on social media.

Luna said the Los Angeles County Sheriff’s office would use every resource, and 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 National Fraternal Order of Police.