The Chiefs have filed tampering charges with the NFL against the Lions, according to a report by Fox Sports reporter Alex Marvez. The exact nature of the complaint is unknown, but it is believed to involve Gunther Cunningham, who was the defensive coordinator in Kansas City from 2004-08 and holds the same position in Detroit currently.
According to the report, Cunnigham may have been in contact with players whom he'd coached with the Chiefs who were set to become free agents in the 2010 offseason, trying to recruit them to the Lions. Detroit didn't end up signing any former Chiefs, however.
If found guilty, the Lions could be fined and/or stripped of at least one draft pick. The NFL, the Chiefs and the Lions all declined comment to Fox.