Arsenal are investigating a video on social media in which a supporter appeared to racially abuse Napoli defender Kalidou Koulibaly.
Unai Emery's men beat their Serie A opponents 2-0 in Thursday's first leg of their Europa League quarter-final at Emirates Stadium.
However, another incident of racism in British and European football marred the match after a clip emerged in which a fan inside the stadium can be heard shouting abuse at Napoli defender Koulibaly.
The same racial slur was also written across the screen to caption the recording and Arsenal confirmed they have started an investigation.
"We utterly condemn the use of any racist language and have launched an investigation to identify the culprit," a club statement read.
"We operate a zero-tolerance approach and anyone behaving like this is not welcome at Arsenal and will be banned from matches.
"We have an extremely diverse community of fans who are all part of the Arsenal family and such incidents are rare at Emirates Stadium.
"We encourage supporters to report any incidents as they happen to stewards or via our matchday alert service."
Elsewhere during Thursday's Europa League fixtures, Chelsea identified three fans shown in footage calling Liverpool's Egypt forward Mohamed Salah "a bomber" and barred them from entering the club's fixture at Slavia Prague.