Uefa has opened disciplinary proceedings following the crowd trouble which overshadowed on Thursday night's Europa League match between Arsenal and Cologne at the Emirates Stadium.
The Group H tie, in which Arsenal eventually ran out as 3-1 winners, was delayed by an hour as ticketless Cologne fans looked to gain entry to the ground.
The visitors were issued with 3,000 tickets, but around 20,000 Cologne fans are believed to have made the journey to London.
There were clearly thousands of away supporters sitting among Arsenal fans, causing a security risk.
Five arrests were made over the course of the evening and Arsenal issued a statement on Friday insisting the safety of supporters was the main concern at the time as the Premier League club "launched a full review" of the events.
Both clubs now face disciplinary charges from governing body Uefa, which include crowd disturbances, setting off fireworks, throwing of objects and acts of damage by Cologne fans, while Arsenal face a probe for "stairways blocked in away supporter sector".
Uefa will deal with the case on September 21, the governing body confirmed in a statement on Friday.
When the game eventually kicked off, Arsenal fell behind early on to a brilliant long-range strike from Jhon Cordoba before bouncing back in the second half, substitute Sead Kolasinac equalising before goals from Alexis Sanchez and Hector Bellerin saw the Gunners to victory.
Afterwards Arsenal manager Arsene Wenger said he was getting ready for the game to be postponed but was uncertain if the delayed kick-off was to blame for the Gunners' poor start.
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"It's difficult to say," he said. "It was similar for Cologne but we had some problems to get off, to start, yes. I don't know, was it mental?
"We waited patiently in our dressing room but what was difficult for me was I had all kind of plans to think about at some stage."
Wenger added: "I had to think if we play tomorrow at what time do we want to play, do we play next week but they had the Bundesliga again midweek and we had a League Cup game midweek as well.
"But in the end at some stage I thought they would not play the game, because I can't see the police taking any risk," he explained.
"We live in a society of 100 per cent security and I thought they would never take a gamble to play this game when I saw the images around the stadium.
"But I must say our supporters dealt well with the situation as well and there was no aggravation."
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Wenger said Cologne fans had been "very clever" to get into areas reserved for home supporters.
"I don't know how they managed to infiltrate our fans and get everywhere but they did that very well. I don't know if they went through Arsenal membership, on the internet... they did very well."
One social media post appeared to indicate scuffles inside the ground, with Richard Conway, a BBC sports news correspondent, tweeting: "Koln [Cologne] fans fighting with stewards inside Emirates stadium. Group arrived in home end and fought their way into away section."
Once the game began, however ...
Cologne manager Peter Stoger refused to be questioned on the actions of the club's supporters.
"I have no comment about the fans," the Austrian said. "I'm the coach, my job is the team, my job is football not the fans."
Cologne are bottom of the Bundesliga table, having lost all three of their opening games in this season's German top flight.
But they qualified for the Europa League after finishing fifth last term.