Arsenal will investigate how vast numbers of Cologne supporters gained access to the home sections of the Emirates Stadium for Thursday's Europa League match, sparking crowd trouble.
Arsenal match delayed by crowd trouble
The Group H opener, which Arsenal won 3-1, was delayed by an hour as ticketless Cologne fans sought to gain entry to the ground.
Visiting fans were issued with 3,000 tickets but around 20,000 fans of the German club are believed to have made the journey to London.
Security risk at Emirates
There were clearly thousands of away supporters sitting among Arsenal fans, causing a security risk.
Five arrests were made and Arsenal issued a statement on Friday insisting the safety of supporters was the main concern at the time.
'Following last night's UEFA Europa League match with Cologne we would like to stress that fan safety was always our paramount concern and informed all decisions made,' it said.
'We have launched a full review into the circumstances surrounding the game and will ensure any lessons that can be learned are used in the future.'
A UEFA probe is also likely after pictures emerged of a minority of Cologne supporters causing problems for stewards inside the stadium.