Three explosions hit German football team Borussia Dortmund's bus late Tuesday ahead of a Champions League home game and police found a letter at the scene claiming responsibility for the attack.

Here is what we know so far:

- Three explosions -

At around 7:15 pm (1715 GMT) three explosions detonated just minutes after Dortmund's team bus left the squad's hotel and headed for their quarter-final, first-leg tie against Monaco at home.

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

A policeman on a motorcycle escorting the team bus suffered trauma from the noise of the blasts.

"The whole team is in a state of shock, you can't get pictures like that out of your head," Dortmund CEO Hans-Joachim Watzke said.

The explosives appear to have been hidden in a hedge and were set off as the bus passed. Investigators believe the bombs may have been homemade using sections of pipes, according to the German daily Bild.

Bild said police were looking for a vehicle that had been registered overseas.

The match was postponed until Wednesday 1645 GMT, with Watzke vowing that his side "will not give in to terror".

- Two claims of responsibility -

German authorities have until now held off from describing it as a terror attack, saying it is too early to determine a motive.

But the probe has now been taken over by federal prosecutors, whose remit includes terror investigations.

A letter found at the scene "claims responsibility for what happened", prosecutor Sandra Luecke said late Tuesday, telling journalists that "its authenticity is being verified".

National media, citing unnamed sources, said investigators were now pursuing a possible Islamist link to the blasts.

The letter referred to the Berlin Christmas market attack in December claimed by the Islamic State extremists group that killed 12 people, as well as Germany's deployment of Tornado reconnaissance missions as part of an anti-IS international coalition.

Separately, national news agency DPA said a second claim of responsibility has emerged online, this time possibly linked to the "anti-fascist" far-left scene.

- Security stepped up -

Dortmund police said security would be tightened at Wednesday's match, with a major deployment of officers ahead of kick-off.

"We are doing everything possible to ensure that the game is played safely," Dortmund's police chief said.

source: AFP