A letter reportedly claiming responsibility for the explosions that went off near the Borussia Dortmund team bus as it transported players to a Champions League quarter-final game is being examined by police, who are so far reluctant to link the incident with terrorism.

The German football team’s quarter-final clash with Monaco had to be abandoned on Tuesday after “an attack, in which serious explosives were used” occurred between Schirrmannweg and Wittbrauckerstrasse in Dortmund around 7 pm local time.

According to German police, three explosives may have been hidden in a hedge close to the official Dortmund team bus.

#Explosion#Dortmund

Unsere Kollegen vor Ort sind weiter im Einsatz. Die Tatortaufnahme ist im vollen Gange. pic.twitter.com/86AAx07gPS — Polizei NRW DO (@polizei_nrw_do) April 11, 2017

Borussia Dortmund has since confirmed that Spanish center back Marc Bartra suffered minor injuries to his right wrist, with the club wishing him a “fast recovery”.

Bei der Explosion wurde @MarcBartra verletzt und befindet sich derzeit im Krankenhaus. Gute und schnelle Genesung, Marc! #bvbasm — Borussia Dortmund (@BVB) April 11, 2017

Diagnose bei @MarcBartra: Speichenbruch und Fremdkörper-Einsprengungen am rechten Handgelenk. Er wird z.Zt. operiert. Gute Besserung, Marc! pic.twitter.com/Ngw7kIdSr0 — Borussia Dortmund (@BVB) April 11, 2017

Thanks for your patience and understanding and the "Dortmund! Dortmund" chants, dear supporters of @AS_Monaco_EN! pic.twitter.com/Gcz9XGQY0J — Borussia Dortmund (@BVB) April 11, 2017

Earlier, the Bundesliga team tweeted that a “bomb blast” had gone off close to the team bus, adding that the players had been taken to safety, while “no threat is posed” to the Westfalenstadion pitch.

Bombenexplosion am Mannschaftsbus am Mannschaftshotel. Spieler in Sicherheit. Keine Gefahr im und am Stadion. Weitere Infos folgen. #bvbasm — Borussia Dortmund (@BVB) April 11, 2017

In an update on the investigation, Dortmund police chief Gregor Lange said a motive for the attack has not yet been established. Meanwhile, a letter found near the bus is being examined by police.

“The risk of a terrorist attack is not new today,” Lange told a press conference on Tuesday night, reported Reuters.

“We have been preparing for this for a long time. I do not want to suggest that this was a terrorist attack. All that is still being investigated. We want to be careful. It is being investigated very professionally.”

BREAKING: A German prosecutor says a letter was found near a soccer team bus where explosives went off; details withheld. — The Associated Press (@AP) April 11, 2017

According to the Associated Press, a German prosecutor said the recently uncovered letter “takes responsibility”for the blasts, although it is not yet known if the document is genuine.

Die Staatsanwaltschaft #Dortmund bestätigt das Auffinden eines Schreibens in der Nähe des Tatortes. Die Echtheit wird derzeit überprüft. — Polizei NRW DO (@polizei_nrw_do) April 11, 2017

The Champions League quarter-final is set to take place on Wednesday, April 12, with Borussia Dortmund encouraging Monaco fans stranded in the city to use the Twitter hashtag #bedforawayfans.

We’re sorry we have to cancel all stadium tours for Wednesday, 12 April. Thanks for your understanding. More info: https://t.co/BlGAqtvYpcpic.twitter.com/IAEiJRxjBh — Borussia Dortmund (@BVB) April 11, 2017