WASHINGTON — President Bush said yesterday that he cannot recall when he learned that Army Ranger Pat Tillman died by friendly fire in Afghanistan in 2004, not at the hands of the enemy as the Defense Department originally claimed.

“I can’t give you the precise moment, but obviously, the minute I heard that the facts that most people believed were true were not true, that I expected there to be a full investigation and get to the bottom of it,” Mr. Bush said in response to a question at a news conference.

Tillman’s death attracted widespread attention because he left a lucrative American professional football career to enlist in the Army after the September 11, 2001, attacks. His comments came as congressional Democrats press an investigation to determine what the White House and top Defense Department officials knew about the circumstances of Tillman’s death, and when. Though Tillman’s direct superiors knew almost immediately that his death was friendly fire, the truth was kept from the public and Tillman’s family for five weeks.

“I always admired the fact that a person who was relatively comfortable in life would be willing to take off one uniform and put on another to defend America, and the best way to honor that commitment of his is to find out the truth, and I’m confident the Defense Department wants to find out the truth, too, and will lay it out for the Tillman family,” Mr. Bush said. House Democrats and Tillman’s family contend that even after seven investigations, the truth remains obscured.

At a hearing of the House Oversight and Government Reform Committee last week, a former defense secretary, Donald Rumsfeld, and several of his top generals denied any cover-up, rejected personal responsibility, and could recall little about how and when they learned of Tillman’s death. Tillman family members say they believe officials at the highest levels of government hid facts to limit public relations damage.

The committee chairman, Rep. Henry Waxman, a Democrat, is pressing the White House for drafts of a speech that Mr. Bush delivered at the White House Correspondents Dinner on May 1, 2004. In the speech, Mr. Bush lamented Tillman’s death but made no reference to the real circumstances of it.

Two days earlier, a top general had written a memo to General John Abizaid, then head of Central Command, warning that it was “highly possible” that Tillman was killed by friendly fire and making clear that the information should be conveyed to the president. The White House has said there is no indication that Mr. Bush received the warning.