A letter found near the scene of a bomb attack on the Borussia Dortmund team bus claims responsibility for the blasts, according to prosecutors.
Police are investigating three "serious explosive devices" which went off as the players and coaching staff left a hotel ahead of a Champions League match in Germany.
Defender Marc Bartra, a Spain international, was taken to hospital after being injured by shattered glass from the coach's windows.
He has had an operation for a broken bone in his hand.
Other players were unhurt, but the match against Monaco was postponed.
Club chief executive Hans-Joachim Watzke said "the team are obviously completely shocked" after what police believe was a targeted attack against the football side.
Police said they believe the explosions were a "targeted attack" against the team and that the devices may have been hidden in a hedge near a parking area.
Mr Watzke said he understood the blasts were by the exit of the hotel and happened as the bus was passing.
A letter that "takes responsibility for the act" was found near the coach and its authenticity is being verified, according to prosecutors.
The team's Swiss goalkeeper, Roman Buerki, was sitting in the back row of the bus, next to Bartra.
He said players ducked for cover, wondering if there would be more explosions.
The blasts happened 10km (6.2 miles) from the Westfalenstadion, where the team had been due to face the French side in a quarter-final first-leg tie.
The game will now be played on Wednesday evening.
Borussia Dortmund said there was no danger at the stadium, where around 80,000 people would have been expected for the game.
The club thanked Monaco supporters for their "patience and understanding" and for chanting "Dortmund! Dortmund!" when the reason for the postponement was announced.
Dortmund recommended that fans stay in the stadium and remain calm to allow an "orderly departure".
Mr Watzke said: "It was all very disciplined.
"I think with all the social media we have these days that people pretty much knew what was going on, and then when the postponement was announced officially."
On social media, Dortmund residents were offering to put up Monaco fans in their homes for the next couple of nights.