Yet Tillman turned down a three-year, $3.6 million contract with the Cardinals enlist. Before starting his military service, Tillman married his high-school girlfriend Marie.

Tillman and his brother went through training to become Army Rangers and were assigned to the second battalion of 75th Ranger Regiment in Fort Lewis, Washington. Tillman served in several tours of duty, including time in Iraq as part of Operation Iraqi Freedom as well as a stay in Afghanistan to serve in Operation Enduring Freedom.

On April 22, 2004, Tillman was killed in action while in a canyon in eastern Afghanistan. The first reports indicated that he was shot during a clash with enemy forces during an ambush. Many questions remained unanswered about Tillman's death at the time, but a week later this account of his death became recognized as the official story, and General Stanley McChrystal approved for the soldier's Silver Star nomination. Pat Tillman was honored in a nationally televised memorial service on May 3, 2004, in which Senator John McCain delivered the eulogy.

Yet there were still many unanswered questions and conflicting accounts concerning the circumstances surrounding his death. As more details emerged, Tillman's family began demanding answers from the military. By the end of May, media outlets reported that Tillman was actually killed in an incident of fratricide—otherwise known as "friendly fire." Official documents would later reveal that the U.S. Army was aware of the possibility of fratricide in regards to Tillman's death even before his memorial service, but withheld that knowledge from the public and from Tillman's family until well after the memorial.

The Pentagon reopened the investigation into Tillman's death in 2005, but the more than 2,000 pages of testimony only revealed more contradictions and inaccuracies. What did become known was that Tillman's platoon was forced to split up when one of their vehicles broke down during a routine search of an Afghan village. Half the platoon members were ordered to tow the vehicle, but were attacked by Taliban insurgents. When Tillman and his half of the platoon came to the rescue, they were mistaken for enemy soldiers. Tillman was shot three times in the head while protecting a young soldier, and two other Americans were wounded.

Documents that surfaced years later also proved that those involved in the incident were aware that Tillman had died from friendly fire within 24 hours of his death—including General Stanley McChrystal, who had approved the Silver Star honor. After Tillman's death, the investigation proved, Army commanders and members of the Bush administration concealed the truth behind the soldier's shooting by destroying items of his clothing, his notebooks, and even hiding parts of Tillman's body to cover up evidence. Even now, the Tillman family remains unsure as to whether the real story of what happened to Pat will ever be fully unearthed. Yet the Tillmans remain persistent in their quest to find out the truth behind Pat's final moments. "This isn't about Pat, this is about what they did to Pat and what they did to a nation," said Pat's mother, Mary Tillman. "By making up these false stories & you're diminishing their true heroism. [The truth] may not be pretty& but that's not what war is all about. It's ugly, it's bloody, it's painful. And to write these glorious tales is really a disservice to the nation."
In addition to his Purple Heart and Silver Star medals from the military, Pat's numbers for the ASU Sun Devils and the Arizona Cardinals were retired in his honor. In June 2010, The N.F.L. and the Pat Tillman Foundation joined forces to create the N.F.L-Tillman Scholarship to honor an individual who "exemplifies Pat Tillman's enduring legacy of service." A documentary about Pat's life, called The Tillman Story, is also slated to hit theaters in August 2010.