Borussia Dortmund and police are investigating a group of people who displayed a banner proclaiming solidarity with an outlawed right-wing group on the Westfalenstadion's huge Sudtribune terrace.
The banner displayed the words "Solidariät mit dem NWDO" ("Solidarity with National Resistance Dortmund"), Der Westen reported. The NWDO, which had strong ties to Germany's right-wing NPD party, was recently outlawed.
Dortmund's fan liaison officer, Sebastian Walleit, said the club had "good video footage" of the banner and those holding it, and had already identified two of them. A spokesman for the city's police told Der Westen: "We have opened investigations."
According to Dortmund Radio 91.2, those identified would be banned from stadiums throughout Germany. Bans would also be imposed on people wearing T-shirts displaying slogans in support of the neo-Nazi Borussenfront faction of supporters.
The banner incident is the second in two competitive matches for Dortmund this season. Before the club's 3-0 cup win over Oberneuland, a Dortmund hooligan group held up a banner reading; "Rico Malt - never to be forgotten."
Malt, who died in 2007 at the age of 32, had strong ties to the right-wing scene, according to Der Westen.
In a statement, Borussia Dortmund's supporters' organisation condemned racism and violence, saying the banner was a source of "shame and consternation" and asking fans not to tolerate it.