The NCAA uses a statute of limitations of four years. Thus, at any time the NCAA can open or re-open an investigation for an infraction occurring within the last four years. However, NCAA convention is to date violations based on when they learned about the infraction. Thus, events that had occurred far more than four years prior to the investigation came under its purview. The initial accident reports revealed several inconsistencies and violations that induced expanded investigation.
This came from the wikipedia page dedicated to the UM hoops scandal. Yes, it's wimkipedia and yes, it's a wound I don't want to relive, but it helps me contextualize some things.