An official with the Riverside Police Department said early Thursday that two of the city's police officers were stopped at a red light when they were "ambushed" by a shooter who is now the subject of an area-wide manhunt.

The officers were partners traveling together in a marked patrol car at Magnolia Avenue and Arlington Boulevard, said Lt. Guy Toussaint.

One of the officers was killed and the other was in surgery early Thursday, officials said. Their names have not yet been released.

PHOTOS: Manhunt for ex-LAPD officer

Authorities said they believe that a Nissan pickup used by a suspect sought by the Los Angeles Police Department, Christopher Dorner, was spotted at the scene.

Dorner is a former LAPD officer wanted in connection with two Orange County slayings. He is believed to have written an online manifesto threatening to harm police officials and their families, authorities said, and is considered "armed and extremely dangerous."

Toussaint said investigators believe that the suspect fled the area and is no longer in the city, but that the department remains on high alert and that police from across the Inland Empire have come in to assist.

"We don't know where he is," Toussaint said. "We are actively looking for him."

He said there was "no indication" that the Riverside police officers "were actively seeking Dorner" when they were ambushed.

"Because of the close proximity and the timeline, we believe there is a strong likelihood ... Dorner is involved in our incident," he said.

When asked by a reporter if police felt as though they were under attack, Toussaint responded: "Based on the circumstances of the shooting, yes, I do."

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-- Phil Willon in Riverside and Kate Mather