Cologne have criticised "inadequate" security measures at the Emirates after crowd issues in their Europa League tie with Arsenal.
UEFA has opened disciplinary proceedings against Arsenal and Cologne following incidents at the Europa League match on Thursday.
Arsenal will hold their own investigation into disturbances after five arrests were made following crowd trouble against Cologne.
Alexis Sanchez's fine strike helped Arsenal to a 3-1 win over Cologne in the Europa League, after the match was delayed by an hour.
Everton endured a torrid start to their Europa League campaign as Atalanta inflicted a demoralising 3-0 defeat on Thursday.
Andre Silva scored a hat-trick as AC Milan cruised to an emphatic 5-1 win away to Austria Vienna in Europa League Group D.
After Arsenal's 3-1 Europa League victory over Cologne on Thursday, we look at four talking points from the Emirates.
Arsene Wenger admits he thought Arsenal's Europa League game against Cologne would be called off after crowd issues.
Alexis Sanchez starts for Arsenal as they host Cologne, but kick-off is delayed for an hour in the "interests of crowd safety".
Arsene Wenger praised the "exceptional" Alexis Sanchez after his fine goal helped Arsenal to a 3-1 Europa League win over Cologne.
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