The Dortmund team bus was attacked as it made its way to the stadium

German prosecutors have doubts about the authenticity of letters that suggested Islamist militants were behind the attack on Borussia Dortmund's team bus on 11 April, with suspicion now turning to right-wing groups.

Three identical letters printed in German found near the scene of Tuesday's attack in Dortmund claimed it was carried out "in the name of Allah".

The letters referred to the use of Tornado reconnaissance planes in Syria, which Germany has deployed as part of the military campaign against Islamic State.

The postponed match was played on Wednesday, Monaco winning 3-2

But a report commissioned by investigators said there were "significant doubts" about the letters and suggested they had been written to trick people into thinking there was an Islamist militant motive.

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

Bild newspaper cited an investigator as saying: "The overall circumstances lead us to believe it's most likely that the perpetrators have a right-wing background."

Marc Bartra was upbeat after the operation on his fractured right wrist (Instagram/marcbartra)

The Dortmund bus was heading to their stadium for a Champions League match against 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.