Prosecutors are investigating a third claim of responsibility for explosions that hit Borussia Dortmund's team bus, according to reports.
Three bombs containing pieces of metal detonated minutes after the bus left the team's hotel for a Champions League match against Monaco.
On Saturday, the Tagesspiegel newspaper reported it had received a claim of responsibility apparently from far-right groups, who hit out at multi-culturalism and threatened another attack.
Frauke Koehler, a spokeswoman for federal prosecutors who was referring to an email received by the newspaper, said: "We have the letter claiming responsibility. We are examining it."
Several claims have already been investigated, but no suspects have been identified.
Three identical letters close to the site of the blasts initially suggested an Islamist link , but questions over the authenticity of the claim have been raised.
One suspect - an Iraqi man - was arrested over the alleged Islamist link but has since been cleared of involvement.
A second claim, purportedly from the far-left, was made online but has also been questioned.
Welt newspaper said on Saturday, citing security sources, that the explosives may have come from army stock.
Spanish international Marc Bartra and a policeman were injured in the blasts on Tuesday.
The team's players were badly shaken by the attack, but played their delayed first leg quarter-final match against Monaco on Wednesday, when they lost 3-2.
Bartra, who had an operation on his wrist after the blasts, was discharged from hospital on Saturday but will be out of action for around four weeks.
The team will host Eintracht Frankfurt in the German Bundesliga on Saturday amid heightened security.