Pentagon officials tell ABC News they believe Iraqi insurgents used a Russian-made SA-7 surface-to-air missile to shoot down a U.S. military helicopter on Monday.
The AH-64 Apache crashed north of Baghdad, killing the two crew members. It was the third American chopper to go down in 10 days.
It's a troubling new development because there are hundreds -- and by some estimates thousands -- of SA-7 missiles that are unaccounted for in Iraq.
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," said Gen. John Keane, the Army's acting chief of staff. "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.
There are also indications insurgents are developing what one Army general calls aerial IEDs -- sophisticated bombs designed to leap in the air and detonate when a helicopter passes by.
Insurgents are believed to be making these devises from remnants of Saddam Hussein's anti-aircraft weapons.
"An aerial IED would be another indication of the enemy's innovation and creativity," said Keane. "Clearly they recognize that the publicity that surrounds a destroyed or damaged aircraft is significantly greater than the publicity that surrounds a destroyed or damaged vehicle."
ABC News' Jonathan Karl filed this report for "World News Tonight."