German prosecutors have cast doubts on the authenticity of letters suggesting Islamist militants carried out the attack on the bus carrying football team Borussia Dortmund to a match.
Three identical letters printed in German found near the scene of the attack on the bus which left one team member injured said it had been carried out “in the name of Allah”, according to broadcaster ARD.
The coach was carrying the UEFA European Champions league team to Signal Iduna Park in the city of Dortmund ahead of a quarter final match against AS Monaco when it was struck by three explosions.
The blasts smashed some of the vehicle’s windows and left Spanish defender Marc Bartra with an injury to his hand and arm.
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," she said of the letters.
Asked why there were doubts, she said she could not give any more information as the investigation was ongoing.
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".
The country is on high alert following the deadly attack on a Berlin Christmas market in December which left 12 people dead.
Amis Amri, a failed Tunisian asylum seeker, ploughed a truck into the crowds at the market next to the Kaiser Wilhelm Memorial Church at Breitscheidplatz on 19 December.
In a separate incident on Friday, German police have arrested three people on suspicion of helping a suspected Islamist militant prepare a bomb attack on police and soldiers, according to German magazine Focus.
It quoted the chief public prosecutor in the northern town of Celle as saying arrest warrants had been issued for two men and one woman suspected of helping preparing a serious act of violence against the state.
The magazine said the trio - a 27-year-old Afghan, a 27-year-old Turk and a 25-year-old German - were arrested on Thursday evening in three different German cities and were in custody.
It said they were suspected of supporting a German member of an ultra-conservative branch of Islam who police arrested in February.
That man admitted planning to lure police or soldiers into a trap and kill them with a homemade bomb.
Additional reporting by Reuters