Tillman walked away from a $3.6 million NFL contract to join the Army after the Sept. 11, 2001, attacks. The Arizona Cardinal safety became a symbol of patriotism to many Americans for his decision.
"While there was no one specific finding of fault, the investigation results indicate that Corp. Tillman probably died as a result of friendly fire while his unit was engaged in combat with enemy forces," Lt. Gen. Philip R. Kensington Jr. said in a statement released by the Army Special Operations Command.
The statement said the firefight took place in "very severe and constricted terrain in impaired light" with enemy combatants firing on U.S. forces.
An Afghan military official told The Associated Press on Saturday that Tillman died because of a "misunderstanding" when two mixed groups of American and Afghan soldiers began firing wildly in the confusion following an explosion.
The Afghan official, who spoke on condition of anonymity, also contradicted U.S. reports that the American soldiers had come under enemy fire.
The official said the two groups had drifted some distance apart during an operation in the remote Spera district of Khost province, close to the Pakistani border.
"Suddenly the sound of a mine explosion was heard somewhere between the two groups and the Americans in one group started firing," the official said, citing an account given to him by an Afghan fighter who was part of that group, not Tillman's.
Tillman's group also opened fire back toward the other group, the official said.
"Nobody knew what it was - a mine, a remote-controlled bomb - or what was going on, or if enemy forces were firing. The situation was very confusing," the official said.
"As the result of this firing, that American was killed and three Afghan soldiers were injured. It was a misunderstanding and afterward they realized that it was a mine that had exploded and there were no enemy forces."
The Army has said the enemy shooters had pinned down other soldiers when.
Tillman's family declined to comment.
The friendly fire was first reported by the Arizona Republic and The Argus of Fremont (Calif.) on Saturday.
"It does seem pretty clear that he was killed by friendly fire," Rep. Trent Franks, R-Ariz., a member of the House Armed Services Committee, told the Republic. Franks said his panel was alerted to the information by the Army's Legislative Liaison Office.
"This does not take away one iota from the heroic nature and courage of the man," the Republic quotes Franks as saying. "The source of that fire is of little consequence in terms of heroism," Franks said. He said that after learning of the Army's conclusions, he made some follow-up inquiries and was satisfied the information was accurate."
Friendly-fire accidents are an inevitable part of warfare, according to an expert cited by the Republic, who agreed that it should not diminish acts of heroism.
"It's tragic, and we probably feel worse about it," Stephen Walt, a professor of International Affairs at the Kennedy School of Government at Harvard University, said to the Republic. "But warfare is a inherently unpredictable activity. Friendly-fire accidents are a part of modern warfare and probably existed as far back as the Stone Age."
The Argus cited a family friend and also government sources it did not name as confirming the friendly fire report.
Tillman turned down a three-year, $3.6 million deal when he enlisted in the Army in May 2002.
He was posthumously awarded the Silver Star, one of the Army's highest honors
Some 110 U.S. soldiers have been killed - 39 of them in combat - since Operation Enduring Freedom began in Afghanistan in late 2001.