As many as three explosions damaged the bus of one of Germany’s most storied soccer teams as it headed to its stadium in Dortmund on Tuesday, wounding one player and forcing postponement of the match, an important playoff in a major European championship.

The Dortmund police chief, Gregor Lange, said at a late-night news conference that “we assume it was a targeted attack” on the Borussia Dortmund team. The wounded player, Marc Bartra of Spain, was undergoing surgery on his right wrist, a spokesman for the team said.

Hans-Joachim Watzke, Borussia Dortmund’s chief executive, said that “three explosive devices” had detonated near a hotel outside Dortmund where the players were staying.

The state prosecutor told reporters that a letter claiming responsibility had been discovered near the site of the blasts, but that it was too soon to say if it was genuine. The prosecutor, Sandra Lücke, declined to answer further questions about the letter’s contents, including what language it was written in.

The authorities also would not describe the three explosive devices. Mr. Lange said that a fourth “suspicious object” had also been found at the scene, but that it had not been set to go off. Photographs of the bus showed that the rear window had been shattered, and the tires appeared to have been blown out.

In an interview with Blick, a Swiss newspaper, the goalkeeper Roman Bürki described the moments after what he said was a “huge bang” as the bus turned onto the main road to go to the stadium.

“I was sitting in the very back row next to Marc Bartra, who was hit by fragments of the broken rear window,” said Mr. Bürki, who is Swiss. All the players then ducked and lay on the bus floor, he said, because “we didn’t know if something more would happen.”

He added: “The police were there quickly and sealed everything off. We are all shocked — in those minutes, no one was thinking about football.”

On Twitter, Prime Minister Mariano Rajoy of Spain wished a “speedy recovery” for Mr. Bartra, 26, who played with Barcelona for seven seasons before joining Dortmund last year.

(The bus’ double glasses protected the team from worse injuries)

The match, against Monaco, will be played on Wednesday night, the first leg of a quarterfinal in the UEFA Champions League.

In a Twitter post, the Dortmund police told fans: “We are preparing for a big deployment and will take care of security” at the match on Wednesday.

Anxiety over terrorism instigated or inspired by the Islamic State and other extremist groups operating in Europe has risen steadily over the past few years after bloody assaults in France, Belgium, Britain and, most recently, Sweden.

In Germany on Dec. 19, a truck crashed into a Berlin Christmas market, killing 12 people.

Soccer has also figured in Germany’s recent brushes with terrorism. The country’s national team was playing France in Paris in November 2015 when the deadliest terrorist attacks in the French capital opened with a suicide bomber blowing himself up outside the stadium.

Days later, the German authorities postponed the national team’s next international match, in Hanover against the Netherlands, after receiving what security officials termed credible threats of an attack. In that episode, thousands of fans had already reached the stadium when the last-minute postponement was announced. Precise details about the tip or any evidence have not emerged.

On Tuesday at the Westfalenstadion, the stadium where the match was to be played, hundreds of Monaco fans, hearing the news, chanted “Dortmund! Dortmund!” in a show of solidarity. In what has become another signal of support when terrorist assaults disrupt people’s plans, Dortmund fans quickly adopted a hashtag offering Monaco fans places to stay.

(Marc Bartra, the player who was wounded, was undergoing surgery on his right wrist)

Chancellor Angela Merkel, an avowed soccer fan, often attends important international matches. Her chief of staff, Peter Altmaier, condemned the attack on Twitter. “A despicable, cowardly deed, whoever did it,” he wrote. He thanked the police, fans and players for staying calm and doing their duty.

Borussia Dortmund is one of Germany’s most successful teams, dating to the late 19th century when soccer first came to Germany from England.

Dortmund, in the heavily populated Ruhr industrial region, is the site of a national museum dedicated to the sport that opened less than two years ago.

The match to be played on Wednesday will demand extra effort from the shocked players, Mr. Watzke said. “The question is, can we forget the scenes of today?” he added.

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