Three explosions on Tuesday evening near the soccer team's bus wounded a Dortmund player and a police officer as they headed to a Champions League match against Monaco.

Federal prosecutors said the 26-year-old Iraqi — identified only as Abdul Beset A. in line with German privacy laws — allegedly joined IS in Iraq by late 2014 and led a unit of about 10 fighters involved in preparing kidnappings, extortions and killings.

He traveled to Turkey in March 2015 and remained in contact with members of IS after arriving in Germany in early 2016, prosecutors said. However, "so far the investigation has turned up no evidence that the suspect participated in the attack," they said in a statement.

A judge later Thursday ordered that A. be kept in investigative detention over his alleged IS membership.

Authorities also searched the home of a second suspected Islamic extremist but prosecutors said Thursday there were no indications he was involved in the bus attack.

Police say the metal-packed devices used in the attack were sophisticated and would have required detailed knowledge of explosives and detonators.

Investigators are still puzzling over three copies of a note claiming responsibility for the attack that were found at the scene.

The note demanded that Germany withdraw reconnaissance jets assisting the fight against IS and close the U.S. Ramstein Air Base in Germany.

Experts say the letter's mix of correct, complicated German and obvious mistakes raises the possibility it is a red herring intended to place blame on Islamic extremists.

The head of the domestic intelligence agency's regional branch, Burkhard Freier, said that Arabic phrases were missing from the note and its demands weren't typical of IS, noting that "IS doesn't negotiate."

He said that Islamic, left-wing and right-wing extremists are being considered by investigators, the news agency dpa reported.