The German FA (DFB) has opened an investigation after Mainz players Leon Balogun and Anthony Ujah said they were racially abused by Hannover 96 fans ahead of last Saturday's Bundesliga match.
On Tuesday, the DFB told ESPN FC it had begun proceedings. Under its rules, the club could face a fine or even a stadium closure, but the governing body said "possible legal consequences are pure speculation at this stage."
Hannover, Balogun and Ujah have been asked to provide written statements, and after the DFB has received them and looked at other material, it will be decided whether investigations continue.
Ujah and Balogun said they had been subjected to racial abuse from the stands when warming up before Saturday's match, and Hannover apologised on Monday.
Balogun wrote on Twitter that "Ujah and I were racially insulted with ape noises among other things" and added that he was "shocked that such behaviour exists in 2018 in the Bundesliga."
"We are dismayed," Hannover sporting director Horst Heldt said on Tuesday, adding that such behaviour "cannot be tolerated, not in a stadium and not elsewhere."
He said: "We are doing everything we can to get to the bottom of it."
Mainz sporting executive Rouven Schroder said on Tuesday that the incidents were "unacceptable, but only carried out by a few" while Hamburg CEO Heribert Bruchhagen added: "We won't be able to fight against scattered idiots."
At the weekend, news wire dpa reported that Hannover had said between six and 12 people were involved in the incident.