Ricardo "Ricky" Galvez, 29, was found dead at the end of his shift in the 10900 block of Brookshire Avenue shortly after 11 p.m.
Lt. John Corina said at least two suspects walked up to the police officer as he sat in the driver's seat of his own vehicle and opened fire.
A fellow Downey police officer heard the gunshots and chased the suspects to the neighboring city of Montebello, where the suspects bailed on foot near Washington Boulevard and Carob Way.
"As luck would have it, a Downey police officer in his patrol vehicle heard the shooting, went in pursuit of the suspects, chased them into the city of Montebello, and that's where they bailed out of the car," Corina said. "And we are currently ... in the process of trying to locate those suspects."
As of 11:30 a.m., at least one arrest had been made. The motive remains unclear but Corina said it appears Galvez was targeted.
"It doesn't look like it was random," Corina said.
Authorities taped off the entire parking lot area, and Downey police officers at the scene appeared to be emotionally shaken.
Galvez had been with the Downey Police Department for five years and had served as a U.S. Marine for two tours, Downey police Chief Carl Charles said at a news conference.
Galvez was as "a tremendous young man who loved serving the residents of Downey," Charles said. "His smile was infectious, and his professionalism was always on display."
Fellow colleagues paid respect to the slain officer as they guided his coffin from the crime scene to the coroner's office.
A large perimeter was set up and evacuations were ordered in the Montebello area where the suspects fled from police.
Resident Victor Bocanegra said police knocked on his door shortly after 2 a.m. and escorted his family out to the street.
"They've evacuated some buildings and some townhomes, and they're looking for this suspect that killed a police officer," he said.
Bocanegra said he saw police put at least two people into patrol cars and drive away.
One suspect was arrested, and several others were being sought, Corina said. He said investigators believe the two individuals involved in the killing were male. No weapon has been recovered, he said.
Washington Boulevard was closed between Paramount Boulevard and Greenwood Avenue for several hours.
The investigation is ongoing. Anyone with information on this case was asked to contact the Los Angeles County Sheriff's Department's Homicide Bureau at (323) 890-5500.