German prosecutors have doubts about the authenticity of letters that had suggested Islamist militants carried out an attack on the bus of soccer team Borussia Dortmund, a spokeswoman said today.

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.

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, ARD added.

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

WATCH: Dortmund players in shock after attack on team bus

"It is indeed doubtful," she said of the letters.

Video: Reuters

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

Three explosions went off as the Borussia Dortmund players' bus travelled to their stadium for a Champions League match against AS Monaco on Tuesday, injuring Spanish defender Marc Bartra.

Experts have been expressing scepticism about the origins of the letters for days. 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".

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