German prosecutors have doubts about the authenticity of letters suggesting Islamist militants carried out an attack on the bus of soccer team Borussia Dortmund.
They say left-wing or right-wing extremists, or violent football fans, could also have been responsible for the attack.
The Borussia Dortmund players' bus were heading to their stadium for a Champions League match against AS Monaco on Tuesday when three explosions occurred, injuring Spanish defender Marc Bartra.
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.
However, a study of the language used in the letters reinforced investigators' earlier doubts about their authenticity.
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.
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 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 April 22.
Koehler said prosecutors had received a copy of the letter but she could not comment further.
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".