Suspected Sunni insurgents penetrated the Baghdad security net, hitting Shiite targets with four bomb attacks that killed 183 people - the bloodiest day since the US troop increase began nine weeks ago. The most devastating blast struck the Sadriyah market as workers were leaving for the day, charring a lineup of minibuses that came to pick them up. At least 127 people were killed and 148 wounded, including men who were rebuilding the market after a Feb. 3 bombing left 137 dead. Wednesday's car bombing appeared meticulously planned. It took place at a pedestrian entrance where tall concrete barriers had been erected after the earlier attack. It was the only way out of the compound, and the construction workers were widely known to leave at about 4 p.m. - the time of the bombing. One builder, 28-year-old Salih Mustafa, said he was waiting for a bus home when the bomb exploded. "I rushed with others to give a hand and help the victims," he said. "I saw three bodies in a wooden cart, and civilian cars were helping to take away the victims. It was really a horrible scene." US military spokesman Maj. Gen. William Caldwell told The Associated Press that al-Qaida in Iraq was suspected in the bombing. "Initial indications based on intelligence sources show that it was linked to al-Qaida," Caldwell said in a late-night telephone interview. Echoing those remarks, US Defense Secretary Robert Gates called the bombings "horrifying" and accused al-Qaida of being behind them. The attacks appeared to be yet another attempt by Sunni insurgents and al-Qaida to force Shiite militiamen back onto the streets. Radical Shi'ite cleric Muqtada al-Sadr had ordered his Mahdi Army fighters to put away their weapons and go underground before the security crackdown began, leaving regions like those bombed on Wednesday highly vulnerable. An outburst of violence from the Shi'ite militia would also ease pressure on the Sunni insurgents, creating a second front for US and Iraqi soldiers struggling to diminish violence in the capital and provide time for the Iraqi government to gather momentum for sectarian reconciliation. US officials have reported a decrease in sectarian killings in Baghdad since the US-Iraqi security crackdown was launched Feb. 14. But the past week has seen several spectacular attacks in the capital, including a suicide bombing inside parliament and a powerful blast that collapsed a landmark bridge across the Tigris River. The number of bodies dumped in the streets of Baghdad also has risen significantly. Brig. Gen. Qassim al-Moussawi, the Iraqi military spokesman, said, "We have not seen such a wave of attacks since the security plan began. These are terrorist challenges. Ninety-five percent of those killed today were civilians." Late Wednesday, Prime Minister Nouri al-Maliki ordered the arrest of the Iraqi army colonel who was in charge of security in the region around the Sadriyah market. The colonel's name was not given. "Our Iraqi people are being subjected to a brutal attack that does not differentiate between an old man, a child or a woman. This targeting of civilian populations brings back to our minds the mass crimes and genocide committed by the Saddamist dictatorial regime," said a statement from al-Maliki's office. The 127 deaths in the market bombing were recorded by Raad Muhsin, an official at the al-Kindi Hospital morgue where the victims were taken. A police official confirmed the toll, speaking on condition of anonymity because he was not authorized to release the information. 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 (11,355 liters) 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 Shi'ite 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-Shi'ite 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.