WASHINGTON -- An Air Force investigation into a shooting spree by an Afghan colonel that left nine U.S. military trainers dead last year found high levels of hostility between Afghan and American forces and major Afghan support for the shooter.

The report released Tuesday by the Pentagon revealed that the Afghan colonel - who methodically opened fire in a conference room at a highly secured Afghan air base in April - told friends that he had returned to Afghanistan from Pakistan "to kill Americans."

Col. Ahmed Gul shot all but one of the Americans in the head, and all at least twice, in what remains the war's deadliest shooting incident targeting Americans. Far from making him a pariah, more than 1,500 Afghans attended Gul's funeral, Air Force investigators reported.

The 436-page report by the Air Force Office of Special Investigations said there was no evidence that Gul, who killed himself after the incident with two self-inflicted shots to the torso, was a member of the Taliban. At the time, a Taliban spokesman took credit for the killings.

But investigators - who interviewed witnesses, other U.S. trainers and Afghan officers and airmen - found that the relationship between Afghan troops and their American mentors is often filled with hostility bordering on hatred. These tensions continue even after a decade in which the U.S.-led NATO coalition has closely trained Afghan forces that it describes as its allies.

Afghan soldiers, at best, tolerated the U.S. troops who were training them to build their own air force, one coalition member told investigators. U.S. troops struggled to gain the trust of their Afghan counterparts and were distrustful of them. Some U.S. troops flashed loaded weapons at Afghans as a show of force, the report said.

One U.S. service member at the scene, in his statement about the shooting, said that his initial reaction was simply: "I knew it."

The report also found that that one of the reasons Gul fired so many shots was that he faced little retaliation. Only one of those killed, Capt. Nathan J. Nylander, 35, of Hockley, Texas, removed his service weapon from the holster and shot back, nearly emptying his gun, the report found. At least one other airman also fired back.

The shootings occurred at the start of a routine meeting held at the base at Kabul's international airport between Afghans and their American trainers. Gul walked up to the back of Maj. David L. Brodeur, 34, of Auburn, Mass., and wordlessly shot him in the head before opening fire on others. Eight U.S. airmen and one American civilian were killed.

Most of those in the room ran away from the scene, the report found. Outside the room, Gul wrote, "God is one" and "God is in your name" in Dari with blood. Gul then fatally shot himself twice in the chest.

The Americans were training their Afghan counterparts on how to use computer systems to better plan out flight missions. One witness said that the training had made only a little progress at the time of the shooting.

Cases of so-called green-on-blue shootings - in which an Afghan attacks international troops - have grown more frequent in recent months. In May, a report commissioned by the U.S.-led coalition and obtained by McClatchy found that such attacks had made up 6 percent of Western deaths in Afghanistan in the previous two years and 30 percent of all field-grade officer deaths.