A note left at the scene of the recent bombing of a bus carrying the German Borussa Dortmund soccer team — which was circulated among ISIS supporters via WhatsApp — suggests a connection between ISIS and the terrorist attack.

On Tuesday, three pipe bombs hit a bus carrying the soccer team as it left its hotel. One player and a police officer were injured in the attack.

A copy of the note circulated on WhatsApp claimed that the attack was in response to Germany’s participation in the fight against ISIS. The note referred to the deaths of 12 “unbelievers” who were killed by “our blessed brothers in Germany.” Britain’s Daily Star newspaper reported that this referred to the December ISIS attack on a Berlin Christmas market.

The most recent note also berates German Chancellor Angela Merkel for continuing to fight ISIS.

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“Your tornadoes (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, athletes and celebrities from Germany and other Western nations are on ISIS’ death list, the note said. If genuine, the note marks the first time that ISIS has publicly threatened the lives of Western celebrities.

The ISIS message also said that Germany must close the Ramstein Air Base, which used by the US. military and end its use of its ‘tornadoes’ in missions against ISIS in Syria.

The bus bombing marked 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. And 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 that the police investigating the bombing 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 has been arrested so far in the course of the investigation.

Other motives for the attack are also being investigated. The note could be “an attempt to lay a false trail,” Ralf Jaeger, the 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, [or] it could be Islamic extremism.”