Baghdad, Iraq ? Violence erupted across northern Iraq on Saturday, as a car bomb killed at least 20 people in front of a police academy and insurgents shot down a U.S. helicopter during an attack on a suspected militant hide-out.

The helicopter’s two crew members were injured when it crash-landed in Tal Afar, a town near the Syrian border, the military said. Both were recovered during a rescue operation in which U.S. troops killed two insurgents, the military said.

The attack on the academy in the northern oil city of Kirkuk, at least the fourth on a police building since the United States turned over sovereignty to Iraqis in June, marked a renewed effort by insurgents to disrupt the training of Iraqi security forces that are supposed to relieve the burden on U.S.-led troops charged with maintaining order here.

The upheaval in the north, a usually quiet region of Iraq, was a reminder that large swaths of this war-torn nation remain dangerous for U.S. troops and Iraqis alike.

U.S. Marines also launched an operation Saturday to clean up a troubled area of south-central Iraq, and saboteurs again attacked the country’s oil infrastructure in the north and south of the country, the military and Iraqi officials said.

In the first incident in the north Saturday, the attacker detonated the car bomb in Kirkuk as hundreds of recruits were leaving the police academy. The blast left behind a scene of twisted cars, smoking rubble and blood-smeared concrete. Police said 35 people were wounded.

Witnesses reported seeing a car approaching about 100 yards away from the academy entrance when it suddenly exploded at around 3:30 p.m. Most of those killed were recruits, but at least one child was among the dead.