BERLIN - Two claims have been found for the responsibility for Tuesday night's attack on a Borussia Dortmund (BVB) team bus.

The German Federal Prosecutor's Office in Karlsruhe has been investigating the attack on the bus. Three explosive devices went off, placed by the side of the road, next to the bus transporting the football players to the match against AS Monaco.

Dortmund's Marc Bartra severely injured his hand during the attack and was rushed to the hospital for emergency surgery on Tuesday night to treat a fracture and to remove shrapnel. A police officer on a motorcycle accompanying the bus suffered acoustic shock and trauma.

A letter claiming responsibility was found close to the bombing location, according to German media sources, claiming the attack was orchestrated by Islamic State (IS) supporters in revenge for the German army's operations in Syria.

The letter also refers to the attack on the Christmas market in Berlin and reportedly starts with: "In the name of Allah."

The letter calls for the German planes to be withdrawn and the US air base in Ramstein to be shut down. The letter is not signed and does not display any symbols of the Islamic State.

It was as yet unclear whether the letter for responsibility originates from the perpetrators of the bomb attack. German police also considered the letter could be a deception designed to mislead them, according to the Sueddeutsche Zeitung.

German Federal Authorities have started a thorough analysis of the letter, involving forensic experts.

A second claim of responsibility posted online linked the attacks to members of the anti-fascist scene. The team's bus is said to be "a symbol for the politics of the BVB which do not sufficiently commit against racists, Nazis and right-wing populists," since for many years "people with misanthropic views are allowed into the stadiums."

Both claims cannot be easily associated with the attackers. According to Spiegel, a high-ranking official from North Rhine-Westphalia stated that on both accounts, there is much reason for skepticism.

"This is a very grave story," said Hans-Joachim Watzke, managing director of the BVB, "what happened today is a new dimension." German Federal Minister of the Interior Thomas de Maiziere said: "My thoughts are with the team. It is now the time to investigate the background. I hope that football will be focus again tomorrow."

The match has been postponed to Wednesday and would take place with an increased police presence.

"We will, of course, be present with increased forces in the stadium and also, of course, to try to do our best to protect both teams," said Nina Vogt, media representative for the Dortmund police.