Dortmund fans open their doors to stranded Monaco supporters

A window of Dortmund's team bus is damaged after an explosion before the Champions League quarterfinal soccer match between Borussia Dortmund and AS Monaco in Dortmund western Germany Tuesday

"Metal strips had been embedded into the explosives and some of that metal found its way into the bus that carried the players of the Borussia Dortmund team", he said.

The match was called off shortly before kickoff and rescheduled for Wednesday evening.

The blast shattered the bus windows and Spanish worldwide Marc Bartra, 26, successfully underwent surgery on a broken wrist after he was hit by flying glass.

Investigators were today examining a letter left at the scene, which reportedly claimed Germany is on an "Islamic death list" because of its military involvement in Syria and referred to the terror attack on a Christmas market in Berlin.

The footballing community too came out in Bartra's support with leading players and clubs sending their regards for the Spanish defender.

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He said: "You could feel the atmosphere change inside the stadium".

Tuesday's encounter was postponed until Wednesday, with UEFA undertaking a review of security procedures for all matches, as Atletico Madrid host Leicester City, and Real Madrid head to Bayern Munich.

Ralf Jaeger, interior minister in the state of North Rhine-Westphalia which includes Dortmund, said the investigation was looking "in all directions", and it was unclear if one or several attackers were involved.

Koehler said there are "significant doubts" about a second claim of responsibility found online suggesting a left-wing extremist motive for the bus attack.

Manchester United playmaker Mkhitaryan, signed from Dortmund last summer, tweeted on Wednesday morning: "Deeply shocked by the explosions last night in Dortmund".

Players ducked for cover, wondering whether there would be more explosions. Police plan to have increased security at the match.

The blasts happened as the vehicle was leaving the team hotel in Dortmund before their Champions League match against Monaco.

The solidarity prevailed as the two teams entered the stadium and prepared for the quarter-final match, with fans from the two teams drinking together and hugging each other.