UEFA has charged Cologne and Arsenal, following crowd trouble during their Europa League game at the Emirates Stadium on Thursday.
The match was delayed by an hour, as thousands of away fans tried to force their way in.
On Friday, UEFA announced that it had opened disciplinary proceedings.
It has brought four charges against Cologne: crowd disturbances, setting off fireworks, throwing objects and acts of damage.
Arsenal face one charge of violating security regulations, because of blocked stairways in the away section.
UEFA said its disciplinary committee would deal with the case next Thursday, with its rules stating that the host club is “responsible for order and security both inside and around the stadium before, during and after matches.”
The rules also say the hosts are “liable for incidents of any kind… unless they can prove that they have not been negligent in any way in the organisation of the match.”
However, the regulations state that each club can be punished for “inappropriate behaviour on the part of their supporters.”
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