Insurgents exploded a truck carrying chlorine gas canisters - the second such "dirty" chemical attack in two days - while a US official said ground fire apparently forced the downing of a Black Hawk helicopter. All nine aboard the aircraft were rescued. The rash of spectacular insurgent attacks offer a sweeping narrative on evolving tactics by Sunni militants 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.