Manchester City say one of their fans is in “a critical condition in hospital” after an alleged assault following their Champions League win over Schalke in Germany on Wednesday.
City won the first leg of the last-16 tie 3-2.
“The club are working with Greater Manchester Police and German Police to ascertain more information,” said City.
“Club staff have remained in Germany to support the family of [the] injured City fan.”
City have asked anyone with any information about the matter to contact police.
The Premier League champions took the lead in the game through Sergio Aguero before Schalke, based in the western German city of Gelsenkirchen, went 2-1 up through two Nabil Bentaleb penalties.
Nicolas Otamendi was sent off for the visitors but goals from Leroy Sane and Raheem Sterling secured victory.