BAGHDAD - Three car bombs tore through Baghdad and the former insurgent stronghold of Fallujah Sunday, killing at least 36 people and breaking what had been a period of relative calm since the end of the Muslim holy month of Ramadan.

The violence was the worst to strike Iraq since the number of American troops in the country dropped below 50,000 and the U.S. declared a formal end to combat operations. It underlines challenges Iraqi security forces face trying to stabilize Baghdad as the U.S. trims its military presence and Iraq's police and military assume security roles.

The deadliest attack Sunday took place in north Baghdad's Kazimiyah neighborhood when a car bomb detonated near a branch office of the National Security Ministry in Adan square, killing at least 21 people and wounding more than 70, police and hospital officials said.

"It was a big explosion and dust and smoke filled my house," said Abu Shahad, who lives about 200 yards from the blast site. He said his cousin and her child were killed and another cousin was wounded in the blast.

At least 10 people were killed in another car bombing in western Baghdad's affluent Mansour neighborhood, said Army Brig. Gen. Ali Fadhal, who is responsible for the western half of the city. Another 10 people were wounded. Fadhal said security officials were investigating whether the blast was the work of a suicide attacker.

Eyewitness Abu Haidar, who works in a nearby office, accused government officials of failing to quell violence in the country.

In Fallujah, a suicide attacker in a car struck an Iraqi army patrol in the city's commercial district, killing one soldier and four civilians, according to police and hospital officials. At least 15 people were wounded in the attack.

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