Police in Southern California say four city pickups have been torched in an attack that may be linked to previous booby-trap attempts to harm officers.

The Hemet city code enforcement trucks were discovered burning at a city lot about 11:30 p.m. Tuesday.

Police Chief Richard Dana says it's considered a threat against the department.

Dana says a 911 caller on Friday warned a police car would be blown up as retaliation against a crackdown on a motorcycle gang.

Officers in Hemet — 85 miles east of Los Angeles — have been the targets of three booby-trap attempts since Dec. 31 but escaped injury. A gas pipe was rerouted into a gang task force building, a security fence was rigged with a gun that went off and an explosive device was attached to an officer's car.

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Information from: The Press-Enterprise, http://www.pe.com