Manchester City are investigating after their Champions League clash with Celtic was marred by crowd trouble.
Hundreds of visiting fans obtained tickets for home areas at the Etihad Stadium, leading to criticism of City by furious sections of their own support.
Proceedings have not been opened against the Premier League club by Uefa, who have charged Celtic over setting off fireworks and crowd disturbances.
However, Etihad officials are looking at how so many visiting fans ended up in areas reserved for home spectators and have vowed to take action against any City fan found to have sold their tickets to Celtic followers.