As first reported by NewsOK.com, Cleveland County District Attorney Greg Mashburn said prosecutors declined to file sexual assault charges against Shannon weeks ago.
He said Shannon was not charged because the woman who filed the complaint did not wish to press charges and based on the facts that were presented.
Furthermore, he said it was not 100 percent what had actually taken place based on the accounts of Shannon and the woman.
Shannon's future at OU will be determined through an appeals process over the allegation he sexually assaulted a female student at his off-campus apartment Jan. 20.
Shannon, the leading tackler for OU last season, was noticeably absent the last week. During Thursday's open practice, linebackers coach Tim Kish said Shannon was ill and couldn't make it.
Shannon also didn't participate in the spring game. Coach Bob Stoops addressed that issue after the game, saying "Frank is working through some personal issues so we will and see on that."
Shannon has been accused of attempting to forcibly have sex with a woman in his bedroom after the two left a party together, according to a Title IX sexual misconduct allegation report obtained by The Oklahoman in Oklahoma City.
Under Title IX, claims of sexual harassment, discrimination, misconduct and assault brought to the attention of most university staff must be reported.
Mashburn told NewsOK.com the police have not re-presented anything to his office, and OU's decision is totally separate from any kind of criminal investigation.
The OU athletic department has declined to comment on the situation Wednesday night and Thursday morning.