Police Col. Saad al-Shamari says the bomb exploded early Saturday outside Fallujah, which is 40 miles (65 kilometers) west of the Iraqi capital.

Iraq’s security forces have become the primary target for insurgents since U.S. troops pulled out of Iraqi cities at the end of June.

Violence has dropped sharply nationwide, but minor attacks continue along with occasional massive bombings. Fallujah is a former stronghold of the Sunni insurgency and the scene of one of the biggest battles of the Iraq war when U.S. Marines wrested control of the city from insurgents in 2004. The city has been relatively quiet since.