Arsene Wenger was surprised Arsenal's Europa League game went ahead after crowd problems caused by ticketless Cologne fans delayed kick-off.
About 20,000 away supporters turned up in London despite the German club being given a ticket allocation of 2,900.
After Thursday's 3-1 win, Gunners boss Wenger said: "I thought they would never take a gamble to play this game when I saw the images."
Arsenal announced they have launched a review into the events on Thursday.
The club statement read: "We worked with our colleagues at Cologne to stop supporters travelling without match tickets.
"The 3,000 tickets issued to Cologne fans was in line with competition rules but it is clear many more visiting fans arrived, causing significant congestion and disturbance outside the stadium before kick-off. Many tickets were sold through touts and this is very disappointing."
The game was the German side's first in European competition for 25 years and while the match itself appeared to pass off without any incidents of crowd trouble, thousands of away supporters ended up in home sections of the stadium.
Arsenal came from behind to win, with Alexis Sanchez scoring his first goal of the season, two weeks after the collapse of his deadline-day move to Manchester City.
Speaking about the delay, which saw the start of the game moved to 21:05 BST, Wenger added: "I don't know how they [Cologne supporters] were able to infiltrate our fans but they did it very well.
"We waited patiently in our dressing room but we had all kinds of plans to think about: do we play tomorrow? Next week?
"It was all kind of speculation that we had to analyse. In the end, at some stage, I thought they would not play the game."
Cologne coach Peter Stoger refused to answer questions on the actions of his club's supporters: "I have no comment about the fans," he said.
"I'm the coach, my job is the team, my job is football, not the fans."
Police said five people had been arrested on suspicion of public order offences.
Former England midfielder Jermaine Jenas told BT Sport: "They weren't nice scenes and there'll be a huge investigation.
"It's worrying that so many away fans were able to get into Arsenal sections. There were Cologne fans celebrating when they scored in the Arsenal areas and anything could have happened - it could have really kicked off."#BBC