BAGHDAD, Feb 14 (Reuters) - A U.S. Marine helicopter that crashed in Iraq last week killing all seven aboard was caused by enemy fire, The U.S. military said on Wednesday, making it at least the fifth such aircraft to be shot down in three weeks. "Initial evidence indicated that the CH-46 Sea Knight helicopter went down as a result of mechanical failure. After further investigation using all available means, the cause of the incident has been confirmed to be hostile fire," Major Jeff Pool, a military spokesman, was quoted in a statement as saying. The U.S. military, which relies heavily on helicopters to transport troops and equipment to avoid Iraq's dangerous roads, has said it is adjusting its tactics in the wake of the unusually high number of helicopters hit in recent weeks by Iraqi militants.
