With Dortmund players due back on the pitch in the Bundesliga, police
are pursuing leads regarding the attack on Tuesday that injured Spanish
international Marc Bartra and a policeman.
Three bombs containing pieces of metal detonated minutes after the
bus left the team's hotel for a Champions League match against Monaco.
The Tagesspiegel newspaper revealed it had received a new claim,
apparently from far-right circles, railing against multi-culturalism,
saying it had been behind the triple blast and threatening another
"We have the letter claiming responsibility. We are examining it," said Frauke Koehler, spokesperson for federal prosecutors, referring to the email received by Tagesspiegel.
Several such claims have already been investigated without any breakthrough.
Three identical letters found close to the site had initially suggested an Islamist link, but questions have arisen about the authenticity of that claim.
A sole suspect - an Iraqi man - taken into custody over the Islamist link has since been cleared of involvement and a second claim, purporting to be from the far-left, was made online, but prosecutors had cast doubt on it early on.
Badly shaken by the attack, Dortmund nevertheless played their delayed first leg quarter-final match against Monaco on Wednesday, losing 3-2 to the French club.
The team is gearing up to host Eintracht Frankfurt in the Bundesliga on Saturday, when six other league games are also to be played under heightened security.
Dortmund urged fans to turn up early due to security checks, and asked them to avoid bringing bags or backpacks.