Jon Krakauer explains:
After Tillman died, the most important thing to know is that within--instantly, within 24 hours certainly, everybody on the ground, everyone intimately involved knew it was friendly fire. There's never any doubt it was friendly fire. McChrystal was told within 24 hours it was friendly fire. Also, immediately they started this paperwork to give Tillman a Silver Star. And the Silver Star ended up being at the center of the cover-up. So McChrystal--Tillman faced this devastating fire from his own guys, and he tried to protect a young private by exposing himself to this, this fire. That's why he was killed and the private wasn't. Without friendly fire there's no valor, there's no Silver Star. There was no enemy fire, yet McChrystal authored, he closely supervised over a number of days this fraudulent medal recommendation that talked about devastating enemy fire.
The only thing less credible is McChrystal's disavowal of his responsibility for some of the worst abuse and torture of detainees in Iraq.