Police are reportedly investigating whether Tuesday night's bomb attack on the Borussia Dortmund team bus has Islamist involvement.
Three bomb explosions shattered the glass windows of the football club's coach while the team were travelling to the Westfalenstadion for their Champions League quarter-final first leg against AS Monaco.
Nobody was seriously injured, but Dortmund defender Marc Bartra required surgery after breaking a bone in his arm and had debris - thought to be broken glass - lodged in his hand.
Local police confirmed that a letter claiming responsibility for the attack was found at the scene, but the authorities have not yet revealed its content.
According to German media outlet Suddeutsche Zeitung, police are investigating a possible Islamist link after the letter reportedly began with the words, "In the name of Allah the merciful, the compassionate".
SZ went on to say that the December attack on a Christmas market in Berlin was also mentioned in the letter, while a threat against Germany and "other crusader nations" was made, claiming that they are on "an Isis death list" until the country withdraws its Tornado military jets from Syria.
However, it is believed that the police are uncertain whether the letter is genuine and are not ruling out the possibility of the piece being used as a decoy.
Meanwhile, the Champions League match between Dortmund and Monaco has been rescheduled for 5.45pm UK time today.
