The network said the shootdown represented "a troubling new development" because there are hundreds and possibly thousands of SA-7 missiles that remain unaccounted for in Iraq.

The AH-64 Apache went down north of Baghdad, killing its two crew members and becoming the third US helicopter to be shot down in 10 days.

According to the report, the weapons had been part of Saddam Hussein's arsenal, much of which was looted after the invasion.

But until now, insurgents had never successfully used them against an American aircraft.

"It could be just a lucky shot," General John Keane, the Army's acting chief of staff, told ABC News. "Or it could be that they have invested in a training program and they now have some qualified operators and that'll be more of a threat than it has been in the past."

Apache helicopters are designed to be able to survive attacks by missiles like the SA-7, but the military is investigating why the chopper targeted in Monday's attack did not, the report said

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