Insurgents exploded a truck carrying chlorine gas canisters Wednesday — the second such attack in two days — while a U.S. official said ground fire apparently forced the downing a Black Hawk helicopter and a rescue mission to save the nine-member crew.

The attacks offer a sweeping narrative on evolving tactics by Sunni insurgents who have proved remarkably adaptable.

Military officials worry extremists may have recently gained more access to firepower such as shoulder-fired anti-aircraft rockets and heavy machine guns — and more expertise to use them. The Black Hawk would be at least the eighth U.S. helicopter to crash or be taken down by hostile fire in the past month.

The gas cloud in Baghdad, meanwhile, suggests possible new and coordinated strategies by bombers trying to unleash toxic — and potentially deadly — materials. "Terrorists are using dirty means," said Brig. Gen. Qassim Moussawi, an Iraqi military spokesman.

Lt. Col. Christopher Garver, a U.S. military spokesman, said initial reports indicated the chopper was brought down by "small arms fire and rocket-propelled grenades" north of Baghdad, but gave no further details. All nine crew members were taken aboard a rescue helicopter, he said.

In Baghdad, a pickup truck carrying chlorine gas cylinders was blown apart, killing at least five people and sending more than 55 to hospitals gasping for breath and rubbing stinging eyes, police said.

On Tuesday, a bomb planted on a chlorine tanker left more than 150 villagers stricken north of the capital. More than 60 were still under medical care on Wednesday. Chlorine causes respiratory trouble and skin irritation in low levels and possible death with heavy exposure.

Some authorities believe militants could be trying to maximize the panic from their attacks by adding chlorine or other noxious substances.

"It is an indication of maliciousness, a desire to injure and kill innocent people in the vicinity," said Garver, who also predicted militants may begin to launch similar attacks because of the widespread mayhem caused by this week's chlorine clouds.

"If there is a particular success, we'll see copycats ... They certainly pay attention to what they think is successful," he said.

In Najaf, the suicide attack killed at least 13 at a police checkpoint. It fit the pattern that's believed to drive much of Iraq's recent violence: Sunni militants seeking to provoke majority Shiites into a full-blown sectarian conflict that would leave Washington's plans in ruins.

It was the first major bombing in more than six months in Najaf, an important Shiite pilgrimage site 100 miles south of Baghdad and also the headquarters of radical cleric Muqtada al-Sadr, head of the infamous Mahdi Army.

An American military spokesman, Maj. Gen. William Caldwell, praised the "incredible restraint" holding back a complete breakdown. But there were fresh blows to the country's fragile political unity.

A rape allegation by a Sunni woman against members of the Shiite-led police force has sent tremors to the highest levels of power. Prime Minister Nouri al-Maliki is now facing mounting accusations of directing a cover-up after he quickly dismissed the 20-year-old woman's claims on Tuesday.

And the U.S. military is caught in the middle. Caldwell said the woman was briefly under the care of U.S. medical personnel after her release from detention Sunday. But he would give no further details and said any hospital records would remain confidential.

"We never release any medical information on patients. Period," Caldwell told reporters.

The Najaf blast hit while streets were filled with morning shoppers. At least seven of the victims were police and the rest civilians near a checkpoint — part of the city's security cordon that includes Mahdi Army militiamen, who battled U.S. forces in the area in 2004.

More than 40 people were injured in the blast, which sent body parts and blood over a wide boulevard. Crews stuffed limbs and bits of flesh into cardboard boxes.

Najaf is a major Shiite pilgrim destination for its Imam Ali shrine near the city's huge cemetery — used as a burial place for Shiites throughout the country. On Aug. 10, a suicide attack near the shrine killed at least 35 people and wounded more than 100.

In Baghdad, another Mahdi Army center was hit. A car bombing in the teeming Sadr City district killed at least three.

More than 10 people died in blasts across Baghdad — adding to the more than 100 victims of bombings in attacks in and around the capital since Sunday. The toll cast a long shadow over authorities marking the first week of the U.S.-Iraqi security sweeps.

Moussawi, the Iraqi military spokesman, said the campaign to reclaim control of the city "has achieved very important goals despite the expected criminal reactions."

"God willing, the plan will continue to uproot terrorists and outlaws across Baghdad and other areas," he told a news conference. He added that 42 "terrorists" have been killed in the sweeps and more than 250 suspected militants arrested, but gave further details.

Caldwell told a news conference that U.S. and Iraqi forces were focusing on "belts" of extremist activity in Baghdad and suggested talks are ongoing over when and how to move into Sadr City, a stronghold of the Mahdi Army militia.

It is believed that al-Sadr has ordered his forces not to challenge the security operation up to this point.

"Anytime you can find a political solution instead of a military one it is better," Caldwell said.

Meanwhile, the U.S.-led coalition in Iraq may soon be shrinking.

A U.S. Marine was killed in fighting in the volatile Anbar province and a soldier was killed by gunfire in a neighborhood of Baghdad, the military said Wednesday.

The Marine was killed Tuesday during combat operations in the insurgent stronghold. The soldier was hit by small arms fire in a northern district of Baghdad on Tuesday, a statement said without giving further details.

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