More than 1,500 Rangers fans infiltrated home areas of Old Trafford, sparking violent scenes at their Europa League clash, Manchester United have found.
The Premier League club launched an investigation after multiple brawls marred the cross-border fixture in January.
And Mail Sport can reveal security bosses refused to turf out visiting fans in home areas on the night for fear of making the situation even worse.
Large numbers of Rangers fans were clearly visible across the stadium during the fixture. But it was only when the Scottish side scored a late equaliser, and those supporters celebrated, that serious trouble flared.