The authors of the email described the Dortmund bus attack as a "final warning" and threatened another strike next Saturday in Cologne, the German "Tagesspiegel" daily reported on Friday.
The message mentions Adolf Hitler and derides multiculturalism, adding that a "Cologne squad" is standing by to spill "colorful blood." This could be a reference to the upcoming pro-tolerance rally in Cologne, where protesters are set to rally against the anti-immigration AfD party. One of the protesters' slogans is "colorful instead of brown," with brown symbolizing fascism, the "Tagesspiegel" said.
German federal prosecutors are examining the email in Karlsruhe, spokeswoman Frauke Köhler confirmed to the DPA news agency.
Fake evidence to mislead officials?
The anonymous note is casting further doubt on preliminary reports that Muslim extremists were behind the bombing that injured two people. Upon inspecting the scene on Tuesday, police found three identical letters that allegedly mentioned the "Islamic State" militia, slammed Chancellor Angela Merkel, and demanded that Germany pull its jets from the campaign in Syria.
However, investigators now have "significant doubts" - the letters could be fabricated to mislead the authorities, German public broadcasters and the "Süddeutsche Zeitung" reported on Friday. The officials also received a claim of responsibility from a left-wing group, but the prosecutors reportedly doubt its authenticity as well.
According to German media, the authorities were also considering the possibility that a foreign security service was behind the bombing. The attacker allegedly used military-grade detonators for the triple blast, which are very hard to come by in Germany.