The FBI just wanted to keep a lid on this. Hinrichs didn't walk seven blocks with a backpack stuffed with TATP in order to commit suicide. And there was a report that he tried to get inside the stadium at one of the gates.
My take is, he was waiting for the crowd to start coming out of the stadium, and had a work-related accident. People need to wake up and stop making excuses.
More than just the FBI. Also Boren and OU folks who saw a lot at risk if people started to think the place was a terrorist target.