Far-right plotters have reportedly claimed responsibility for the attack on the Dortmund Borussia team bus which wounded defender Marc Bartra.
An anonymous email sent to Berlin's Tagesspiegel daily newspaper said the triple blast was intended to be a "final warning" against multicultural policies.
It cited Adolf Hitler and warned of a potential further attack, the newspaper said.
Federal prosecutors are now examining the message, the latest in a number of claims made over Tuesday night's triple explosion on the bus.
Police had earlier revealed that a letter was found at the scene suggesting an Islamist extremist was responsible, but they had "significant doubts" over its credibility.
Prosecutors have questioned the authenticity of another claim posted online, suggesting a left-wing extremist motive.
One Iraqi man has been arrested over the attack, who has been named as 26-year-old Abdul Beset A.
German authorities say he is believed to be a former commander with an elite unit in the so-called Islamic State - but there is no evidence to link him to the attacks on the Dortmund team.
Spanish defender Marc Bartra suffered injuries to his wrist and arm caused by shattered glass from the explosion, which happened about 90 minutes before Dortmund were due to play Monaco at the nearby Westfalenstadion.
He has since described the attack as the "hardest 15 minutes" of his life.