Tillman, who served with the Army Rangers, was 27.

Although the military had not officially confirmed his death, the White House put out a statement of sympathy that praised Tillman as "an inspiration both on an off the football field."

Former Cardinals head coach Dave McGinnis said he felt both overwhelming sorrow and tremendous pride in Tillman, who "represented all that was good in sports."

"Pat knew his purpose in life," McGinnis said. "He proudly walked away from a career in football to a greater calling."

Several of Tillman's friends have said the Sept. 11, 2001, terrorist attacks influenced his decision to enlist.

Lt. Col. Matt Beevers, a spokesman for the U.S. military in Kabul, confirmed that a U.S. soldier was killed Thursday evening, but would not say whether it was Tillman.

He said the soldier died after a firefight with anti-coalition militia forces about 25 miles southwest of a U.S. military base at Khost, which has been the scene of frequent attacks.

Two other U.S. soldiers on the combat patrol were injured, and an Afghan soldier fighting alongside the Americans was killed.

Arizona Sen. John McCain noted that Tillman declined to speak publicly about his decision to put his National Football League career on hold.

"He viewed his decision as no more patriotic than that of his less fortunate, less renowned countrymen who loved our country enough to volunteer to defend her in a time of peril," McCain, a Republican, said in a statement.

U.S. officials, who spoke on the condition of anonymity, said a formal announcement was expected later in the day. Spokesmen at the Pentagon and U.S. Army declined comment.

Tillman played four seasons with the Cardinals before enlisting in the Army in May 2002. The safety turned down a three-year, $3.6 million deal from Arizona.

He made the decision after returning from his honeymoon with his wife, Marie.

Tillman's brother, Kevin, a former minor league baseball prospect in the Cleveland Indians' organization, also joined the Rangers and served in the Middle East. They committed to three-year stints in the Army.

Some 110 U.S. soldiers have died - 39 of them in combat - during Operation Enduring Freedom, which began in Afghanistan in late 2001.

Tillman's agent, Frank Bauer, has called him a deep and clear thinker who has never valued material things.

In 2001, Tillman turned down a $9 million, five-year offer sheet from the Super Bowl champion St. Louis Rams out of loyalty to the Cardinals, and by joining the Army, he passed on millions more from the team.

Tillman turned aside interview requests after joining the Army. In December, during a trip home, he made a surprise visit to his Cardinal teammates.

"For all the respect and love that all of us have for Pat Tillman and his brother and Marie, for what they did and the sacrifices they made ... believe me, if you have a chance to sit down and talk with them, that respect and that love and admiration increase tenfold," McGinnis said at the time.

It was not immediately clear when he went to Afghanistan.

The 5-foot-11, 200-pound Tillman was distinguished by his intelligence and appetite for rugged play. As an undersized linebacker at Arizona State, he was the Pac-10's defensive player of the year in 1997.

He set a franchise record with 224 tackles in 2000 and warmed up for last year's training camp by competing in a 70.2-mile triathlon in June.

Tillman carried a 3.84 grade point average through college and graduated with high honors in 3 1/2 academic years with a degree in marketing.

"You don't find guys that have that combination of being as bright and as tough as him," Phil Snow, who coached Tillman as Arizona State's defensive coordinator, said in 2002. "This guy could go live in a foxhole for a year by himself with no food."