The vast majority — more than 80 percent — of American military deaths in Iraq are still being caused by Sunni insurgents, according to an ABC News analysis of data released for the month of October by the Defense Department. Of the 99 American soldiers killed in hostile action, at least 81 were killed by IED’s or hostile fire in areas that are dominated by Sunni Arabs and where U.S. forces have been battling Sunni insurgents since the U.S.-led invasion of Iraq. By contrast, only three Americans were killed in areas dominated by Shiite Arabs. Fifteen were killed in central Baghdad where populations are mixed with Sunni and Shiite Arabs, and it is therefore difficult to determine the identity of the attackers. THE BLOTTER RECOMMENDS The Man Who Killed 31 GIs This Is Malibu Calling — Is This Cher? Click Here to Check Out More of the Brian Ross Page Counter-insurgency analysts tell ABC News that the high percentage of deaths in the Sunni areas represents compelling evidence that Sunni insurgents remain the primary threat to the security of U.S. personnel. "If American troops are getting killed in predominantly Sunni areas, then they are getting killed by Sunni insurgents," Caleb Carr, Professor of Counter-Insurgency at Bard College, told ABC News. IEDs continue to be the leading killer of Americans — 65 percent, 42 U.S. soldiers were killed by roadside bombs. Thirty-four Americans were killed by "small arms fire," "hostile fire" or snipers. U.S. military officials attribute the October death toll, which is the highest since January 2005, to several factors, including stepped-up U.S. counter-insurgency operations in Baghdad. Forty-three U.S. soldiers were killed in Baghdad and surrounding environs, more deaths than any other area in Iraq. The majority, 67 percent of those killed, were in Sunni-controlled areas. Despite the geographic data, U.S. military officials seemed unprepared or unwilling to describe which groups were causing the majority of U.S. casualties. When asked whether Americans were being killed primarily by Sunni or Shiite militants, Lt. Col. Christopher Garver, Director of the Combined Press Information Center for the Multi-National Forces in Iraq, said that he did not have the breakdown. "I would not want to categorize that…I would just be guessing," he said. Counter-insurgency experts dismissed the notion that Shiite extremists could be infiltrating the Sunni areas to kill Americans. "The one thing we know for sure in the Muslim world in general, not just Iraq, is that everybody knows what the radicals are up to," said Professor Carr. "There are no hidden Shiite activists hiding out in Sunni areas because the populations won’t tolerate it."