An insurgent videotape obtained by ABC News shows nearly a dozen sniper attacks targeting American troops.
"He definitely knows what to do with a rifle," said Maj. John Plaster, a retired Green Beret sniper instructor and author of "The Ultimate Sniper." "And he has the judgment and discipline to take a shot, wisely choose an escape route, and immediately depart to avoid capture. This is not a zealot; this is a calculated shooter."
The video, distributed on the Internet and on DVDs sold in Baghdad, credits a lone gunman who calls himself "Juba the Baghdad Sniper." He claims to have killed 143 U.S. service members. It is impossible to verify that claim.
Since the U.S. invasion, more than 370 U.S. troops have been killed by gunshot wounds, and more than 1,000 have been injured. But those figures also include ordinary gun battles.
The U.S. military said Juba does not exist. They claim he's an urban legend, perhaps a composite of several snipers.
But the video makes the threat crystal clear -- even with a flak jacket, helmet, or armor plating, the marksman finds the weak spot.
"The big concern," said Plaster, "is that there's a school somewhere that's ready to turn out more of these people."
U.S. military officials refused to comment for this report. They are concerned that if they acknowledge the threat, it could inspire others.
ABC News' David Wright filed this report for "World News Tonight."