Borussia Dortmund’s Champions League match against Monaco had to be postponed after the German club’s bus was damaged by an explosion.Police confirmed there were three explosions near the bus at about 7:15pm local time in Dortmund’s Hochstem district as the team prepared to make its way to the Westfalenstadion.Dortmund defender Marc Bartra went to hospital, where he underwent surgery to repair a broken arm and remove debris lodged in his hand.Bartra’s teammate, goalkeeper Roman Burki, was sat next to the defender when the blasts went off.“After the bang everyone in the bus got their heads down,” Burki told Swiss newspaper Blick, via The Guardian. “We didn’t know whether there would be more. The police arrived quickly, sealed everything off.“We are all in shock.”Police are still trying to discover who was responsible for the attack, which is being invested as attempted homicide, according to Sky Sports’ Kaveh Solhekol.

Suspicious objects found at the scene

But a letter has been found near Dortmund’s bus that “takes responsibility for the act,” according to a German prosecutor.

The Mirror report that police are investigating the authenticity of the letter. They believe that the person or persons responsible was probably an expert in building and detonating bombs.

Police also confirmed that more “suspicious objects” were found at BVB’s hotel.

A drone was used to assist in the search around the hotel.

'Sophisticated expertise in bomb building'

A police spokesman, Gunnar Wortmann, said: “We will be working throughout the night to discover who was behind this attack. All police and vehicles in Dortmund have been mobilized.

"Three explosions detonating at the exact moment the bus passed by suggest a sophisticated expertise in both bomb building and detonation - perhaps using a mobile phone or a garage door-opening device.”

Police believe it was a targeted attack

It is thought that the bombs were hidden in a hedge near a parking lot. But the police’s investigation is still ongoing.

“At this time, it is still not clear what the real background to this act is,” Dortmund police chief Gregor Lange told a news conference, via the Independent.

“We must start from (the position this was) a targeted attack on BVB's team. Strong police forces are in operation.

“We also have explosive detection dogs in use and decided to search the scene from the air. At the present time it is not clear what the background of the act is.”