German police have found a letter left near to the site where Borussia Dortmund's team bus was damaged by three explosions and are investigating a possible Islamic extremist motive behind the attack.
According to local newspaper Süddeutsche Zeitung the letter starts by making reference to "Allah, the most beneficent, the most merciful" and later labels sports stars belonging to "crusading nations" as targets for killing.
Police are also said to be considering the possibility that the document was a false flag intended to mislead.
Dortmund's Champions League quarter-final tie against Monaco, postponed following the attack which left Spanish defender Marc Bartra in hospital with injuries, has been rescheduled to take place tonight.
"I have spoken to the players and told them to show everybody that we don't bow to terror on society," said Dortmund chief executive Hans-Joachim Watzke.
"This is perhaps the most difficult situation we've had to deal with in recent decades. Today we're not playing for us, we're playing for everyone. Whether Borussian, Bavarian or Schalke. And of course for Bartra."
Police have confirmed that the explosions on the bus were a "targeted attack" and that they are investigating the authenticity of the letter left near the scene.