Roman Burki speaks on Borussia Dortmund bus attack

Photo taken on April 11, 2017 shows the German soccer team Dortmund's bus after explosions in Dormund, Germany.

The team was set to play against Monaco in the "Champions League quarterfinal first leg game", the AP reports. Authorities confirmed they found a letter near the scene and are now investigating it.

"Its authenticity is being verified", prosecutor Sandra Luecke told journalists, without giving details on its contents. German authorities have held off from describing it as a terror attack, saying it is too early to determine a motive.

Spanish defender Marc Bartra was injured and the Champions League clash with AS Monaco was postponed by a day until Wednesday. One police officer, who was escorting the bus on a motor cycle, suffered a blast trauma.

Watzke added that there was "no alternative" to rescheduling the match for Wednesday, as Monaco also has to play over the weekend and the return Champions League match is scheduled for next week.

Dortmund police said in a message on Twitter: "After the initial investigation, we assume that this was an attack using serious explosives".

"After the bang, we all crouched down in the bus. Anyone who could lay down on the floor", he continued.

Goalkeeper Roman Burki was sitting at the back of the team bus alongside Bartra, and told Swiss newspaper website blick.ch how events unfolded.

The team later said Bartra was in surgery for a broken radius - a bone in the forearm - and for "bits of debris lodged in his hand". Security was heavy after the explosions and the stadium was evacuated.

"In extreme situations, all Borussia move even closer together and I'm sure the team will feel that tomorrow".

Police added: "Currently there is no evidence of a threat to the visitors at the stadium".

The match has now been rescheduled for Wednesday night 6:45pm local time.

Federation Internationale de Football Association president Gianni Infantino condemned the incident, while Uefa counterpart Aleksander Ceferin said he was "deeply disturbed" and praised the decision to postpone the game.

"The players will be able to push this out of their minds and be in a position to put in their usual performances", he said. "You can imagine yourself whether they slept well or badly", Rauball said.

Borussia Dortmund said there was no danger at the stadium, where around 80,000 people would have been expected for the game.

"Mentally and psychologically that is hard to absorb, it's a lot to deal with". The explosions came "as the players were leaving their hotel for the match at 7 p.m. local time", the wire service writes.