German police express doubt about letters claiming Dortmund attack

German prosecutors have doubts about the authenticity of letters that had suggested Islamic State group (IS) militants carried out an attack on the bus of soccer team Borussia Dortmund, a spokeswoman said on Friday.

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 outlets.

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 group.

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.

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

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

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 22 April.

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

The Borussia Dortmund players' 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 militants 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".

Bartra has said he experienced the "hardest 15 minutes" of his life after fracturing his wrist in the attack on the team bus.

"The pain, the panic, and the uncertainty of not knowing what was going on, nor how long it would last ... they were the longest and hardest 15 minutes of my life," the Spanish international wrote on Instagram on Friday.

Meanwhile, German police have arrested three people on suspicion of helping a suspected Islamist militant prepare a bomb attack on police or soldiers, magazine Focus said on its website.

It cited the chief public prosecutor in the northern town of Celle as saying on Friday that arrest warrants had been issued for the two men and one woman suspected of helping to prepare a serious act of violence against the state.

German authorities have been on high alert since December, when an Islamist truck attack on a Berlin Christmas market killed 12 people.

There was no suggestion in the Focus report that the three suspects arrested were linked to either the Berlin or the Dortmund attacks.