Police have found a letter claiming responsibility for what they are describing as a "targeted attack" on the Borussia Dortmund team bus.

The vehicle was damaged as three explosions occurred while the coach was making its way from the team hotel in the Hochsten area of Dortmund to Signal Iduna Park for the Champions League quarter-final first leg with Monaco.

Defender Marc Bartra was injured in the incident, with the club later confirming that he had been taken to hospital to have surgery on his wrist.

Police were unable to specify what type of explosives were used but they described the devices as "serious", stating they believed they had been hidden under a hedge next to a car park.

Speaking at a news conference, public prosecutor Sandra Lucke confirmed that the incident was being treated as attempted murder and gave assurances that additional security would be in place on Wednesday, with the match rescheduled for 18:45 local time.

Police chief Gregor Lange said: "This evening, there were three explosions at 7:15 in the area surrounding the hotel in which BVB had been a guest.

"We assume that it was a targeted attack on BVB's team. The services were tasked with securing the crime scene on a large scale.

"We decided at a very early stage not to rule anything out. At the present time, the concrete background for this act is not yet clear.

"I would like to express thanks for the good co-operation with BVB. We're preparing for a large-scale operation tomorrow [Wednesday]. The protective measures for the teams have already begun. Bomb detection dogs are still in use."

Lucke added: "The investigation is being carried out based on suspicion of attempted murder. A letter was found near the crime scene. We are checking its authenticity intensively.

"We found someone taking responsibility for the attack in the letter. Due to the investigation, we cannot comment further on this.

"Another object that did not explode was found. Our current point of view is that it was a dummy."