DORTMUND, Germany — Three explosions went off near the Borussia Dortmund team bus as it set off for a Champions League quarterfinal match Tuesday night, slightly injuring a player.

Christian Pulisic, one of the top U.S. national team members, plays for Borussia Dortmund, one of the best teams in Germany.

Police said in a statement they were working on the assumption that the blasts ahead of the team’s match against Monaco were caused by “serious explosive devices,” which may have been hidden in a hedge near a car park.

They didn’t elaborate on the possible nature of the devices or say who might have planted them before the first-leg match, which was called off and rescheduled for Wednesday.

A German prosecutor said a letter found outside the hotel the team bus was departing from when the explosions happened “takes responsibility for the act.”

The prosecutor, Sandra Luecke, said authorities won’t give details of the letter at this stage, citing the ongoing investigation.

Police said the explosions near the Dortmund team bus came as it left the hotel on the outskirts of the city for the stadium, about six miles away, at around 1 p.m. EDT.

A window on the bus was damaged and Spanish defender Marc Bartra was injured.

Dortmund said Bartra was taken to a hospital. Dortmund chief executive Hans-Joachim Watzke said Bartra was injured in the arm and hand “but nothing life-threatening.”

Dortmund goalkeeper Roman Buerki said the team bus had just pulled out of the hotel driveway when an explosion – a “huge bang” – happened and sent glass flying. Buerki told a Swiss daily, Blick, that he was sitting in the last row of the bus, next to Bartra.

Bartra was hit by shards from the broken back window, he said. Players ducked for cover, wondering whether there would be more explosions.

“We’re all shocked. Nobody thought about a football match in the minutes after that,” he said.

Inside the packed stadium, supporters of Monaco, which plays in the French league, chanted “Dortmund, Dortmund” in sympathy for the German side. Dortmund residents, for their part, used social media to offer accommodation to stranded Monaco supporters ahead of their rescheduled match in Europe’s premier soccer club competition.

“The team is totally shocked, that’s clear. It’s our task now to digest this somehow because it’s only 24 hours before we have to play. That’s our job,” Watzke said.

He added there was “no alternative” to rescheduling the match for Wednesday. Monaco also has to play this weekend and the return Champions League match is scheduled for next week.

“It’s a very unfortunate situation but there was no other way,” he said.

A stadium spokesman told fans of the cancellation, saying “there is no reason for panic here at the stadium.”

Dortmund recommended that fans stay in the stadium and remain calm to facilitate an “orderly departure.”

Germany has seen matches postponed over security concerns before.

In Hannover, in November 2015, Germany’s friendly against the Netherlands was canceled just before kickoff after police feared an explosive device might be detonated.

It came days after devices were detonated outside the Stade de France in Paris as France was playing Germany, as part of a coordinated attack on the French capital.