BAGHDAD — Three car bombs tore through Baghdad and the former insurgent stronghold of Fallujah on 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 in Iraq since the number of American troops in the country dropped below 50,000 and the United States declared a formal end to combat operations. It underlines the challenges Iraqi security forces face trying to stabilize Baghdad as Iraq's police and military assume responsibility for security.
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. "I went out and saw a big black cloud hanging over the area where the bomb exploded, and I rushed there because I have relatives living there."
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 in the attack.
Fadhal said security officials were investigating whether the blast was the work of a suicide attacker in a car targeting a crowded commercial area.
The blast sheered off large sections of the concrete walls from the surrounding buildings, and chunks of rubble were strewn around the street. Dozens of Iraqi army and police officers cordoned off the area, keeping journalists at bay.
There was no immediate claim of responsibility for the attacks.
In Fallujah, a suicide attacker in a car struck an Iraqi army patrol in the city's busy commercial district, killing one Iraqi soldier and four civilians, according to police and hospital officials. At least 15 people were wounded in the attack.
Fallujah, 40 miles west of Baghdad, is a former stronghold of Iraq's Sunni insurgency.
Earlier Sunday, two people in a minibus were killed when a roadside bomb went off in the Shula neighborhood of northwestern Baghdad, police said.