An Iraqi man held by German police following the bomb attack on a Borussia Dortmund team bus is alleged to have led an Islamic State unit before travelling to Europe.
The 26-year-old man identified only as Abdel Beset al-O is said to have commanded 10 militants involved in kidnapping, extortion and killing.
However federal prosecutors admitted on Thursday they have no evidence the arrested man took part in the attack, and it remains unclear who was responsible.
Prosecutors on Thursday declined to press charges against the suspect in connection with the blasts and instead said they want to indict him with commanding an Islamic State in Iraq and Levant (Isil) unit in Iraq from 2014.
German prosecutors often charge terror suspects with lesser offences so they can be held while police pursue investigations.
Abdel Beset al-O, who came to Germany in 2016 via Turkey, is believed to have been under covert surveillance as a suspected jihadist for some time.
He was initially held over an intercepted telephone conversation a few days before the attack, in which an unidentified caller told him: “The blast is ready”, according to an unconfirmed report in Spiegel magazine.
A second man who was briefly held has been released without charge. According to local media reports, the 28-year-old German Muslim was sleeping with his child when police raided his home, and was only suspected because he had an umbrella from the Borussia Dortmund team hotel.
At the same time, police are also investigating whether the attackers were from Germany’s far-Left or far-Right movements.
Experts have cast doubt on the authenticity of a letter found close to the site of the attack which claimed responsibility in Isil’s name.
“Much of it simply doesn’t fit with Isil,” Prof Peter Neumann of King’s College, London told Bild newspaper.
The letter demands Germany withdraw aircraft providing reconnaissance for international air strikes against Isil and close Rammstein, an airbase used by the US.
“The demands are very strange,” Prof Neumann said. “Isil never makes such concrete demands or talks so small. Rather they say ‘We will fight till we have annihilated you all, and Isil controls the world’.”
The language used in the letter is also untypical of Isil, according to Prof Neumann.
It has been suggested the letter may be a forgery or a genuine claim of responsibility from Islamic extremists inspired by Isil but not directly linked to the group.
The attack, using remote control bombs detonated by mobile phone, was considerably more sophisticated than recent terror incidents such as those on Westminster Bridge and a Berlin Christmas market.
A second letter which was found claiming responsibility in the name of “Antifa” far-Left extremists has been largely discounted as a more obvious forgery.
But Bild reported on Thursday police have identified two other possible suspects, one from the far-Left and one from the far-Right.
There have even been suggestions of a possible link to football hooliganism. Borussia Dortmund as involved in an incident in Februray in which a small number of its fans threw stones at supporters of rival club Leipzig.
Police have refused to comment other than to say they are continuing to investigate in all directions.
“Every lead will be followed. There is a very wide investigation but it will take time,” Sebastian Fiedler, deputy head of the national police union said. “You can’t expect results in a day.”