Lofton had one of the most productive seasons ever by an Oklahoma linebacker in 2007 and watched his draft stock increase.
Faced with a decision, Lofton distanced himself a week ago and had a four-hour meeting with coaches on Saturday.
With the information, Lofton decided Sunday afternoon not to return.
"I'm leaving!" he told Sooners Illustrated.
Lofton earned consensus All-American and Big XII Defensive Player of the Year honors while recording 153 tackles, the most of any player in the Bob Stoops era.