PHOENIX (Reuters) - An Arizona detention officer who worked for the jail system run by America's self-described "toughest sheriff" was shot and killed in the driveway of his Phoenix home as he prepared to go to work before dawn on Thursday, authorities said.

Police officers using dogs quickly mounted a two-hour search of the residential neighborhood after the shooting of the officer, who authorities said had been wearing clothing with a sheriff's office insignia on it. There was no immediate word on motive.

"We will certainly look into whether it was related to his duties, but at this point we don't know," Phoenix police spokesman Sergeant Trent Crump said. No arrests have been made.

Witnesses reported several shots fired at about 4 a.m. local time and spotted a man fleeing the scene on foot, police said. Television footage from the scene showed a pick-up truck in the driveway with its hood up and the driver's door open.

When authorities responded to the scene, they found the officer severely wounded in front of the home. He was later pronounced dead at a local hospital. His identity was not immediately released.

Sheriff's spokesman Chris Hegstrom said the officer had worked for the Maricopa County Sheriff's Office for eight years and was assigned to the custody support bureau but could not be more specific. The Arizona Republic reported that he worked in the jail system's food factory.

Maricopa County Sheriff Joe Arpaio has been a lightning rod for controversy over his aggressive enforcement of immigration laws. The six-term sheriff has been the subject of several investigations and lawsuits stemming from his get-tough policies.

(By David Schwartz. Editing by Cynthia Johnston and Steve Orlofsky)