However they said that they were seeking an arrest warrant to keep the 26-year-old Iraqi national, identified only as Abdul Beset A. in detention over allegedly having been a "member of the ISIS group" in Iraq.
Abdul Beset A. was detained in connection with the three explosions that rocked the Borussia Dortmund bus late Tuesday, injuring a player and a police officer.
Federal prosecutors have called it a "terrorist" attack and said they are focusing on suspects in the "Islamist spectrum".
But in a statement Thursday they acknowledged: "The investigation has not found evidence that the suspect took part in the attack."
They said, however, that they now believe Abdul Beset A. joined the ISIS group in Iraq in late 2014 and was the commander of a unit of around 10 fighters.
"The goal of the unit was to prepare kidnappings, abductions, extortion and killings," the prosecutors said.
He crossed the border into Turkey in March 2015 and continued on in early 2016 to Germany "where the suspect maintained contact with ISIS members," they added.
Abdul Beset A. will appear Thursday before a judge who will decide on prosecutors' application for the arrest warrant.
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