German police are investigating a possible Islamic extremist link to the bombing of the Borussia Dortmund football team’s bus, German media say.
A letter found near the scene references the Berlin Christmas market attack and military operations in Syria.
It is not yet clear if the letter is genuine.
Player Marc Bartra underwent an operation after breaking a bone in his wrist. No other players were hurt.
German newspaper Suddeutsche Zeitung said the letter, beginning with the phrase “in the name of Allah”, mentioned Germany’s use of Tornado jets in the coalition forces fighting so-called Islamic State (IS).
IS said it carried out the attack on a Christmas market in Berlin.
But it is possible the perpetrators are deliberately trying to mislead the investigation, Suddeutsche Zeitung reported, adding that an analysis of the letter by experts is under way.
Germany’s federal prosecutor, which usually assumes responsibility for terror-related crime, has taken over the investigation.