Baghdad ? Twin suicide car bombers struck police checkpoints at bridges Friday in a predominantly Shiite area of Baghdad, killing at least 23 people just hours after a series of U.S. raids on car bomb networks across the capital killed four suspected insurgents.

The blasts underscored the difficulty U.S.-led forces are having in destroying the presumably Sunni insurgent cells that have stepped up car bombings in the capital since the U.S. began its security crackdown 12 weeks ago.

In all, at least 52 Iraqis were killed or found dead Friday in politically related violence.

The suicide car bombers struck about 6 p.m. The driver of a sedan waiting in a line of cars at a police checkpoint near the old Diyala Bridge blew up his vehicle, partially collapsing the span, police said.

About two minutes later the driver of a large fuel truck barreled toward a second checkpoint at the nearby new Diyala Bridge and set off his explosives, police said. The bridge also was damaged, and firefighters struggled to extinguish burning police and civilian cars that had been driving across during the attack.