The U.S. military announced the capture Thursday of two suspects in the recent spate of helicopter downings in Iraq, as coalition officials vowed to fight back after so-called "dirty" chemical attacks signaled a change in insurgent tactics.

Army Lt. Gen. Ray Odierno, the No. 2 American commander in Iraq, did not offer any details about the suspects but said he believed "Al Qaeda-associated cells" were behind the attacks in which eight helicopters were either shot down or landed under fire in the past month.

Odierno also said the military has noted similarities in tactics and techniques used and that officials were studying them closely to try to better protect the aircraft.

Coalition officials, meanwhile, said a raid on five buildings near Fallujah uncovered three vehicle bombs that were being assembled with about 65 propane tanks and "all kinds of ordinary chemicals."

Maj. Gen. William Caldwell, the chief U.S. military spokesman, said he believed the insurgents were going to try to mix the chemicals with explosives.

Caldwell said a change in tactics by the insurgents to create high-profile attacks was backfiring by turning public opinion against the insurgents. The number of tips provided by Iraqis over the last six months had doubled, Caldwell said.

Thursday's raid came a day after insurgents blew up a truck carrying chlorine gas canisters, killing five people and sending to hospitals more than 55 people gasping for breath and rubbing stinging eyes.

• Visit FOXNews.com's Iraq Center for more in-depth coverage.

Brig. Gen. Qassim Moussawi, an Iraqi military spokesman, said the investigation into the attack was still under way.

"But what is obvious to us [is] that the terrorists are adopting new tactics to cause panic and as many casualties as they can among civilians. But our plans also are always changeable and flexible to face the enemies' new tactics."

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.

In the volatile Sunni stronghold of Ramadi, U.S. troops battled insurgents in fierce fighting that killed at least 12 people. Iraqi authorities said the dead included women and children.

The six-hour firefight began after the U.S. troops were attacked by insurgents with small-arms fire and rocket-propelled grenades Wednesday evening in eastern Ramadi, said Marine spokesman 1st Lt. Shawn Mercer.

The fighting ended after "precision-guided munitions" damaged a number of buildings being used by the insurgents, he said. Twelve insurgents were killed and three were wounded, he said, adding that there were no civilian casualties.

However, Dr. Hafidh Ibrahim of the Ramadi Hospital said 26 people, including four women and children, were killed when three houses were damaged in the fighting.

Ramadi, the provincial capital of the Sunni insurgent stronghold of Anbar, has seen some of the bloodiest street battles of the war.

Separately, four Iraqi soldiers have been accused of raping a 50-year-old Sunni woman and the attempted rape of her two daughters in the second allegation of sexual assault leveled against Iraqi forces this week, an official said.

Brig. Gen. Nijm Abdullah said the attack allegedly occurred about 10 days ago in the northern city of Tal Afar during a search for weapons and insurgents.

The Associated Press contributed to this report.