German investigators were hunting Wednesday for possible suspects responsible for three explosions that rocked the Borussia Dortmund football team’s bus, injuring a player and a policeman.

The assault, described by Dortmund city’s police chief as a “targeted attack” against the team, shook German football ahead of crucial Champions League ties.

Investigations will focus on a letter claiming responsibility for the attack that was found close to the site of the blasts.

“The letter claims responsibility for what happened,” prosecutor Sandra Luecke said late Tuesday, telling journalists that “its authenticity is being verified”.

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

But Germany has been on high alert since a series of jihadist attacks last year, including the Christmas market truck assault in Berlin in December that killed 12 people.

The explosives detonated minutes after the Dortmund team bus pulled away from the squad’s hotel and headed for their quarter-final, first-leg, tie against Monaco.

Spanish international Marc Bartra underwent surgery on a broken wrist after he was hit by flying glass. The injured policeman, who was on a motorcycle and had been escorting the team bus, suffered trauma from the noise of the blasts.

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