(Releads with judge)
BERLIN, April 13 (Reuters) - A German judge authorised on Thursday the arrest of a 26-year-old Iraqi man detained after an attack on a bus carrying players of a top soccer team and prosecutors said they believed he was a member of Islamic State.
The Federal Prosecutor's Office said in a statement there was no evidence that the man, identified as Abdul Beset A. and who arrived in Germany via Turkey at the start of 2016, had taken part in the attack in the western city of Dortmund.
Three explosions went off as the Borussia Dortmund bus travelled to the club's stadium for their Champions League match against AS Monaco on Tuesday, injuring Spanish defender Marc Bartra and delaying the fixture by a day.
The suspect will appear before a judge who will decide whether to approve the arrest warrant issued by prosecutors, which would allow the man to be held for longer than 24 hours.
"The investigations have so far resulted in no evidence that the suspect had taken part in the attack," the prosecutor's office said in a statement.
It added that the man was a member of Islamic State in Iraq, where he led a 10-member commando unit that was involved in kidnappings, smuggling, extortion and killings. The jihadist group controls parts of Iraq and Syria.
Bild newspaper said on Thursday that military detonators had been used to set off the three explosive devices, probably remotely using a mobile phone.
Three identical letters printed in German found near the scene of the attack in Dortmund suggested a possible Islamist motive for the attack.
One of the letters, published by Bild, starts with the Islamic phrase "In the Name of God, the Most Gracious, the Most Merciful" and demands the withdrawal of German Tornado surveillance planes from Syria and the closure of the U.S. military air base in Ramstein, Germany.
"As of now, all infidel actors, singers, athletes and all prominent personalities in Germany and other crusader nations are on a death list of the Islamic State," the letter said.
Bild said prosecutors were also investigating two additional suspects in the attack, one a neo-Nazi sympathiser and another from a radical leftist group. (Additional reporting by Rene Wagner; Editing by John Stonestreet and Gareth Jones)