Arsenal have taken immediate steps to review the fan trouble which disrupted Thursday's Europa League match against Cologne.
The game was delayed by an hour after up to 20,000 visiting fans travelled to Emirates Stadium despite the Bundesliga club receiving a ticket allocation of just 3,000.
The Gunners say "extensive steps" were taken to prevent Cologne fans acquiring tickets for the home sections, but many of the travelling supporters were seated alongside their Arsenal counterparts across all tiers of the ground.
A statement on the club's website said: "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.
"We worked in full consultation with police and UEFA officials on the night and in advance of the game and had taken extensive steps in advance to prevent tickets being sold to visiting supporters."