In the memo sent to a superior officer seven days after Tillman's death, Lt. Gen. Stanley McChrystal warned that the evidence strongly pointed to friendly fire and the nation's leaders risked embarrassing themselves if they publicly said otherwise.
"I felt that it was essential that you received this information as soon as we detected it in order to preclude any unknowing statements by our country's leaders which might cause public embarrassment if the circumstances of Cpl. Tillman's death become public," McChrystal wrote.
The April 29, 2004, memo, was addressed to Gen. John Abizaid, head of Central Command, and was intended as a warning to Bush and acting Army Secretary Les Brownlee.
It is not clear whether Bush or Brownlee received the warning, but it raises new questions on how high up the chain of command the misinformation campaign extended. In speeches following the memo, Bush avoided any reference to the circumstances of Tillman's death.
The family was not told until May 29, 2004, what really happened. In the intervening weeks, the military continued to say Tillman died under enemy fire, and even awarded him the Silver Star, which is given for heroic battlefield action.
White House spokesman Blain Rethmeier said Friday that a review of records turned up no indication that the president had received McChrystal's warning.
Rethmeier emphasized that the president often pays tribute to fallen soldiers without mentioning the exact circumstances of their deaths.
The Tillman family has charged that the military and the Bush administration deliberately deceived his relatives and the nation to avoid turning public opinion against the war.
Tillman's mother, Mary, said Saturday the newly disclosed document demonstrates Bush was complicit in deceiving her family.
"He knew it was friendly fire in the very beginning, and he never intervened to help, and he essentially has covered up a crime in order to promote the war," Mary Tillman said in a telephone interview. "All of this was done for PR purposes."
The memo was provided to the AP by a government official who requested anonymity because the document was not released as part of the Pentagon's official report into the way the Army brass withheld the truth. McChrystal was the highest-ranking officer accused of wrongdoing in the report, issued earlier this week.
In his memo, McChrystal said he had heard Bush and Brownlee "might include comments about Cpl. Tillman's heroism and his approved Silver Star medal in speeches currently being prepared, not knowing the specifics surrounding his death."
McChrystal said he expected an investigation under way "will find that it is highly possible Cpl. Tillman was killed by friendly fire."
At the same time, McChrystal said: "The potential that he might have been killed by friendly fire in no way detracts from his witnessed heroism or the recommended personal decoration for valor in the face of the enemy."
A former spokesman for Abizaid did not immediately return phone and e-mail messages.
As for Brownlee, he told investigators he did not recall learning Tillman was killed by his fellow Rangers until several weeks after the fact. He did not discuss the matter with the White House, he told investigators.
A spokesman for McChrystal said he had no comment. McChrystal was, and still is, commander of the Joint Special Operations Command, head of "black ops" forces. He has since been promoted to lieutenant general. Abizaid was in charge of American forces in the Middle East and Central Asia.