BAGHDAD — The U.S. military blamed a renegade Shiite group Wednesday for a deadly truck bombing in a Baghdad Shiite neighborhood and said it was seeking to re-ignite sectarian violence. Iraqi officials said the death toll from the bombing rose to 63.

The Iraqi government said Tuesday's attack, the deadliest in Baghdad in three months, would stiffen its resolve "to defeat the terrorists and to maintain the security achievements."

No group claimed responsibility for the blast, which occurred on a bustling commercial street in Hurriyah, scene of some of the bloodiest sectarian slaughter in 2006. That led to speculation Sunni extremists may have been behind the attack.

But U.S. spokesman Lt. Col. Steven Stover said the command did not believe al-Qaida in Iraq was behind the attack based on the type of vehicle and explosives used.

Instead, he said the command believed the attack was carried out by a Shiite special group led by Haydar Mehdi Khadum al-Fawadi, whom Stover described as a "murderous thug" seeking to incite violence "for his individual profit and gain."

U.S. and Iraqi forces have been searching for al-Fawadi for months, and his photo is displayed on checkpoints in the area. Authorities have offered a $10,000 bounty for information leading to his arrest.

"We believe he ordered the attack to incite (Shiite) violence against Sunnis; that his intent was to disrupt Sunni resettlement in Hurriyah in order to maintain extortion of real estate rental income to support his nefarious activities," Stover said in an e-mail.

Several Iraqi police officials said the casualty toll stood at 63, with another 78 wounded. They spoke on condition of anonymity because they were not supposed to release the information. Stover gave a figure of 27 dead and 40 wounded.

The blast was the deadliest attack in Baghdad since March 6, when a pair of bombs detonated in the mostly Shiite district of Karradah, killing 68 people and wounding about 120.

The U.S. uses the term "special groups" to identify breakaway factions of the Mahdi Army, the biggest Shiite militia led by anti-American cleric Muqtada al-Sadr. The U.S. says special groups are backed by Iran, although Stover did not mention Iran in his statement.

He said the bomb was believed to have contained 200 to 300 pounds of an undetermined explosive. He also said a U.S. military team was attending a meeting about 150 yards away at the time of the blast.

The blast shattered the relative calm in the capital since a May 11 cease-fire ended seven weeks of fighting between U.S. and Iraqi forces and Shiite militants in the Sadr City district. Ironically, it came the same day the Iraqi parliament announced plans to move outside the U.S.-protected Green Zone — a show of confidence that the worst of Baghdad's violence was over.

In a statement, Prime Minister Nouri al-Maliki's Cabinet said the blast was aimed at raising the morale of extremist groups that have suffered setbacks in Baghdad, Basra and Mosul.

"This crime will not influence our determination and resolve to defeat the terrorists and to maintain the security achievements," the statement said. "Moreover, it will increase our resolve to save the capital and the provinces from terrorists, killers, and outlaws."

The U.S. Embassy and the U.S. military command issued a joint statement condemning the assault as barbaric and pledging to work with Iraqi security "to find those who perpetrated this horrific attack and help bring them to justice."

Iraqi officials have been eager to promote a sense of confidence among the war-weary Iraqi people after months of declining bloodshed in the capital.

Deputy parliamentary speaker Khalid al-Attiyah told lawmakers Tuesday that they will move from the convention center in the Green Zone to the Saddam Hussein-era National Assembly building for their next legislative term, which begins Sept. 1.

But residents of Hurriyah said security around their neighborhood on the west side of the Tigris River had increased in recent days, with new blast walls erected at the entrance to the area and more police and military checkpoints.

On buses Wednesday, Hurriyah commuters heading for work talked about the bombing and speculated what group may have been responsible. Some people said they thought Sunni militants from nearby Adil may have been to blame. Thousands of Sunnis fled Hurriyah for Adil during the height of the sectarian slaughter.

"I think that car bomb came from the Adil area," said Jaafar Ali, 33, a government employee. Ali predicted a mortar attack would be launched on Adil in retaliation, and said the explosion was "meant to raise tension between these two neighborhoods."

U.S. commanders have warned repeatedly that the relative peace in Baghdad is fragile because extremists, including al-Qaida in Iraq and Shiite militant groups, remain capable of high-profile attacks.

The U.S. hopes security measures which have tamped down violence since last summer will be enough to prevent extremists from mounting a sustained campaign of bombings against civilians that could provoke a return to sectarian reprisal attacks.

Survivors complained that the army and police had failed to protect them.

"The blast occurred because there wasn't any security presence by the Iraqi army or police at the scene, not even any checkpoint," said Khalid Hassan, 40, who suffered shrapnel wounds and burns. "People were confused, upset and running in all directions. We are all victims of terrorism and carelessness."