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 fans turned up in London despite the German club being given a ticket allocation of 2,900.
After Thursday's 3-1 win at Emirates Stadium, Gunners boss Wenger said: "We live in a society of 100% security.
"I thought they would never take a gamble to play this game when I saw the images around the stadium."
The game was Cologne's first in European competition for 25 years and while the match itself appeared to pass off without any further 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.