German investigators have "significant doubts" that the attack on the Borussia Dortmund team bus on Tuesday was the work of "Islamists".

Three identical letters printed in German found near the scene of Tuesday's attack in Dortmund had stated it was carried out "in the name of Allah", broadcaster ARD reported, citing other media.

The letters referred to the use of Tornado reconnaissance planes in Syria, which Germany has deployed as part of the military campaign against the Islamic State of Iraq and the Levant (ISIL, known as ISIS).

But a report commissioned by investigators said the letters had likely been written to trick people into thinking there was an ISIL motive, ARD added.

READ MORE: Marc Bartra injured in Borussia Dortmund bus blasts

"That's accurate," Frauke Koehler, a spokeswoman for the federal public prosecutor's office, said when asked to comment on the ARD report.

Asked why there were doubts over the letters, she said she could not give any more information as the investigation was ongoing.

Another letter

German newspaper Tagesspiegel said on its website later on Friday it had received an anonymous far-right email claiming responsibility for Tuesday's attack. It said the email referred to Adolf Hitler, railed against multiculturalism and suggested another attack might occur on April 22.

Koehler said prosecutors had received a copy of the letter but she could not comment further.

READ MORE: Police detain Borussia Dortmund bus attack suspect

The Borussia Dortmund team bus was heading to their stadium for a Champions League match against AS Monaco on Tuesday when three explosions occurred, injuring Spanish defender Marc Bartra.

Experts have been expressing scepticism for days about the origins of the letters claiming responsibility. Security sources have said investigators are looking at whether left or right-wing extremists may have carried out the attack.

A day after the attack, the interior minister of the German state of North Rhine-Westphalia described the letters as "unusual".