The U.S. military on Wednesday weighed into the politically explosive case of a Sunni woman allegedly raped last weekend by three Iraqi policemen, announcing its own investigation after the Shiite-run government dismissed her allegations as false.

The announcement, made to reporters by the chief military spokesman, appeared aimed at containing the growing political storm. Prime Minister Nouri al-Maliki’s exoneration of the three officers after an investigation lasting less than a day has inflamed Sunni-Shiite tensions over a case that strikes at the heart of Iraqi attitudes toward protection of women.

Al-Maliki, a Shiite, stoked the political flames further Wednesday by firing a top Sunni official who called for an international investigation into the woman’s allegations, which were broadcast Monday by satellite television stations across the Middle East.

Rape is considered not only an assault on the victim but a grave offense against her entire family and community.

Al-Maliki insists the charge was fabricated by Sunni politicians and extremists to discredit the police and the security crackdown in Baghdad. He announced a “reward” for the officers who were implicated.

Regardless of the truth, many Sunnis considered the government’s speed in clearing the policemen as an insult to their community. Al-Maliki announced an investigation Monday evening and cleared the officers the next morning.

With the issue threatening to spiral out of control, the U.S. military announced Wednesday that Gen. David Petraeus, the new top U.S. commander in Iraq, had ordered his own investigation, appointing an American officer to begin collecting evidence.

The 20-year-old woman told Arabic language television stations that she was detained Sunday by Iraqi police at her west Baghdad home and accused of aiding Sunni insurgents. She was then taken to a police garrison where she was raped by the three policemen before American soldiers arrived and took her away, she said.

Black Hawk downed

Insurgents exploded a truck carrying chlorine gas canisters Wednesday – the second such “dirty” chemical attack in two days – while a U.S. official said ground fire apparently forced the downing of a Black Hawk helicopter north of Baghdad. All nine aboard were rescued.

The attacks show 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.

The explosion of pickup truck carrying chlorine gas cylinders killed at least five people and sent more than 55 to hospitals.