Police officers stand in front of the Borussia Dortmund team bus an explosion near their hotel before the game. (Martin Meissner/AP)

German investigators said Wednesday that letters uncovered after explosions targeted a Dortmund soccer club suggest that the attack could have links to Islamist extremists, whose demands include closing a U.S. air base in Germany.

Authorities said they detained one person “from the Islamist spectrum” and searched the apartment of a second suspect after Tuesday’s blasts, which injured Borussia Dortmund defender Marc Bartra.

The three explosions went off near the hotel where Borussia Dortmund players and coaches were staying as the team bus was on its way to a home match against AS Monaco in the Champions League. The match was postponed.

Frauke Köhler, a spokeswoman of Germany’s federal public prosecutor, said that three identically worded letters suggesting an Islamist motivation had been found at the scene. The documents demanded the withdrawal of the German Tornado jets supporting the fight against the Islamic State in Syria and the closure of the U.S. air base in the German town of Ramstein-Miesenbach.

Earlier, a statement surfaced online claiming responsibility and suggested a link with left-wing extremism, but Köhler doubted its authenticity.

Should the Islamist motive be confirmed, it would bring a new twist to the terrorist threat in Germany and the first such attack in the country specifically targeting soccer, the nation’s most popular sport.

“We have assumed from the beginning that this is a targeted attack on the BVB team,” Dortmund Police Chief Gregor Lange said at a news conference Tuesday night.

The blasts broke windows on the bus, and shards hit Bartra, who underwent surgery. One police officer, who was escorting the bus on a motorcycle, suffered a blast trauma.

As a sign of solidarity, Interior Minister Thomas de Maizière planned to attend the rescheduled game Wednesday amid ramped-up security. Monacco won, 3-2.

Government spokesman Steffen Seibert condemned the attack as “appalling” and said Chancellor Angela Merkel had spoken on the phone with Borussia Dortmund chief executive Hans-Joachim Watzke.

Seibert praised the BVB fans, who had spontaneously taken to social media Tuesday to offer followers of the French club places to stay for the night.

“The investigating authorities . . . will do everything necessary to find the perpetrator or the perpetrators and bring them to justice,” Seibert told reporters in Berlin.