BAGHDAD – Insurgents in Baghdad set off a chemical “dirty bomb” Wednesday, the second straight day that the new and deadly tactic was used against Iraqi civilians.

At least three people were killed and dozens injured when a truck carrying canisters of chlorine gas exploded at a fuel station in the Bayaa neighborhood of western Baghdad.

A dirty bomb is designed to spread hazardous materials along with the blast. Experts are worried that terrorists may someday release radioactive material with such a bomb. But in these recent cases in Iraq, the hazardous substance has been chlorine, which burns the skin and can be fatal if inhaled in a concentrated form.

On Tuesday, a tanker truck filled with explosives and chlorine erupted on a roadside near Taji, north of the Iraqi capital, killing nine and wounding more than 100, Iraqi officials said. On Jan. 28, a similar truck bomb involving chlorine killed 16 in Ramadi.

The dirty bomb Wednesday and other recent attacks in Baghdad seemed tailored to shake confidence in the government and the U.S. and Iraqi forces that are combing the city under a much-anticipated security plan that began a week ago.

The Baghdad crackdown has spawned a divisive controversy involving a Sunni woman’s allegations that she was raped by Iraqi police, most of whom are Shiite.

On Wednesday, Prime Minister Nouri al-Maliki fired the head of the influential Sunni Endowment, who had called for an international investigation of the Iraqi police over the rape allegations.

Speaking to Arabic-language television stations from Amman, Sheikh Ahmed Abdul-Ghafour al-Samarraie said he refused to step down from his influential post on procedural grounds. He was selected for his post by the country’s president, not its prime minister.

The firestorm over the alleged rape began Monday when the woman appeared on Al Jazeera television, claiming she had been taken from her Baghdad home by police and repeatedly assaulted.

Al-Maliki called for an immediate investigation, but just as quickly issued a second public statement asserting that the woman’s claims were proven false by a medical examination.

Government officials said the woman had been put up to making the accusations by Sunni political blocs opposing the prime minister.

Though violence is now rampant in Iraq, it is unusual in the Middle East for women to speak publicly about being sexually assaulted, a crime generally considered a mark of shame for her family.

On Wednesday, the U.S. military found itself in the middle of the battle between Sunni and Shiite politicians over the rape allegation.

Maj. Gen. William Caldwell told reporters that the woman had been treated in an American hospital in Baghdad’s Green Zone, and said her medical records were given to her when she left.

But he said they were not released to the Iraqi government, which seemed to contradict al-Maliki’s claims. Caldwell said Gen. David Petraeus, the new U.S. commander in Iraq, had ordered a military lawyer to investigate the accusation, but would not say whether the woman had been sexually assaulted, citing American patient privacy laws.

Caldwell said the accusations should not cause Iraqis to distrust the government or lose faith in the Baghdad security plan.

“These allegations are made against individuals, not against the government of Iraq,” Caldwell said. “Obviously this needs to be looked into very carefully. But that should not be connected with this operation.”

So far, elements of three Iraqi Army brigades have been added to the force in Baghdad, he said. An extra U.S. brigade of some 2,700 troops also has moved in, and there are plans to bring a total of about 17,500 American troops by the end of May.

Parts of the American units will be quartered alongside Iraqi soldiers in “joint security stations,” urban encampments in vacant homes that house up to 200 troops. Fourteen such stations have already been established around Baghdad, Caldwell said. As many as 30 eventually will be set up, according to the military.

Around the city, stepped-up security operations continued, with spokespeople for both country’s forces on Wednesday announcing the capture of car bomb makers and leaders of “rogue” Mahdi Army militia cells, as well as seized weapons caches and re-opened neighborhood markets.

Elsewhere, insurgent bombings killed 13 at the gates to the Old City in the Shiite holy city of Najaf, while a pair of bombs caused injuries in a parking garage in the ethnically diverse northern city of Kirkuk.

Also Wednesday, the military announced the death of a U.S. soldier in northern Baghdad on Tuesday. The U.S. military death toll in the war stands at 3,146.