Notes left at the scene of Tuesday's bombing of a bus carrying the German Borussa Dortmund soccer team and circulated among ISIS supporters via WhatsApp suggest a connection to the terror group.
Three pipe bombs hit the bus as the team left its hotel before a Champions League match. One player and a police officer were injured and the bus's glass shattered.
A copy of the note circulated on WhatsApp claimed the attack came in reaction to Germany's participation in the fight against ISIS. It refers to the deaths of 12 "unbelievers" who were killed by "our blessed brothers in Germany." Britain's Daily Star newspaper reports this refers to the December attack on a Berlin Christmas market.
It berates German Chancellor Angela Merkel for continuing to fight ISIS.
"Your tornados (German jet fighters) are still flying over the Caliphate to murder Muslims. But we will remain steadfast because of Allah's mercy," the note said.
"Unbelieving" actors, sportsmen and celebrities from Germany and other Western nations are on ISIS's death list, the note said. If genuine, this marks the first time ISIS has publicly threatened the lives of Western celebrities.
Germany must close Ramstein Air Base used by the U.S. military and end its use of its Tornados in missions against ISIS in Syria, the note said.
The bus bombing marks the third ISIS-related terrorist attack in Europe in the past month. Last Friday an ISIS inspired attacker plowed a truck into a building in Stockholm, Sweden, killing four people and wounding 13 others. On March 22, an ISIS-inspired attacker drove a car into pedestrians near London's Palace of Westminster, the seat of the British parliament, killing five people and injuring 49 others.
Frauke Koehler, a spokeswoman for German federal prosecutors, said they were focusing on two suspected Islamic extremists and have searched their homes. They are also investigating the credibility of the notes, she said. One suspect was arrested.
Other motives are being investigated. The note could be "an attempt to lay a false trail," Ralf Jaeger, German Interior Minister for the North Rhine-Westphalia state, told the Associated Press.
"We are investigating in every direction, and it's really meant that way," Jaeger said. "It could be left-wing extremism or right-wing extremism. It could be the violent fan scene, it could be Islamic extremism."