Besides the market attack, bombs struck Shiite targets in the capital at a police checkpoint, near a hospital and in a small bus.

Nationwide the number of people killed or found dead was 233, which was second only to a total of 281 killed or found dead on Nov. 23, 2006. Those figures are according to AP record-keeping, which began in May 2005.

Caldwell said militants were "attempting to destroy any sense of security the people of Baghdad were beginning to feel with the security operation in Baghdad."

He called insurgents a "vicious cancer on the body of Iraq. You've got to keep fighting it. We're not going to give up."

Many of the most devastating bomb attacks in the country have come in the past several months, indicating insurgents have developed more sophisticated or powerful explosives.

US military officials announced that last week they found 3,000 gallons of nitric acid hidden in a warehouse in downtown Baghdad. US forces discovered the acid, a key fertilizer component that can also be used in explosives, during a routine search April 12, the military said.

Timothy M. Swager, head of the chemistry department at the Massachusetts Institute of Technology, said that aside from being used to make explosives, nitric acid could cause dangerous burns if used directly on people.

"Like all strong acids, if you sprayed people directly with it would burn them very badly," he said.

Steve Kornguth, director of the biological and chemical defense program at the University of Texas in Austin, said nitric acid is less toxic than chlorine gas at the same concentration, but could also be lethal.

He said in his opinion, insurgents are probably "experimenting with different ways of releasing harmful materials as an indirect effect of explosions."

Hospital officials have been reporting more serious burn victims, both among the dead and wounded, in recent attacks.

About an hour before the market was hit, a suicide car bomber crashed into an Iraqi police checkpoint at an entrance to Sadr City, the capital's biggest Shiite Muslim neighborhood and a stronghold for the Mahdi Army militia.

The explosion killed at least 41 people, including five Iraqi security officers, and wounded 76, police and hospital officials said.

A towering column of black smoke rose from a tangle of eight incinerated vehicles that were in a jam of cars stopped at the checkpoint. Bystanders scrambled over twisted metal to drag victims from the smoldering wreckage. Iraqi guards who survived the bombing staggered through the carnage, apparently stunned.

During the noon hour, a parked car exploded near a private hospital in Karradah, a predominantly Shiite district in the center of Baghdad. At least 11 people died and 13 were wounded, police said. The blast damaged the Abdul-Majid hospital and other nearby buildings.

The fourth bombing exploded in a small bus in the central Rusafi area, killing four people and wounding six, police said.

In other violence, a suicide bomber struck a police patrol at nightfall in the Saydiyah neighborhood, a mixed Sunni-Shiite district in southwest Baghdad. Four died, including two policemen, and eight were wounded, five of them police, police officials said.

The US military also said a suspected insurgent was killed and eight captured in two raids north of Baghdad on Wednesday. Some of the suspects were believed linked to al-Qaida in Iraq and to a militant cell that has used chlorine in truck bombings, the statement said.

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