BERLIN — A letter that was found after explosions damaged the team bus of one of Germany’s premier soccer teams called for the country to scale back its involvement in the Western military coalition in Syria, the authorities said on Wednesday.

Frauke Köhler, a spokeswoman for the Federal Prosecutor’s Office of Germany, also said that two people with an “Islamist background” had been taken into custody after an attack on the Borussia Dortmund bus Tuesday evening when the team was traveling to its stadium for a Champions League match against A.S. Monaco.

The game was postponed and kicked off Wednesday night at a packed stadium in Dortmund under tightened security.

Ms. Köhler said the letter demanded that Germany withdraw its Tornado aircraft from the campaign in Syria, where they are used for reconnaissance and where the Islamic State is under attack from a multinational coalition trying to push it from its strongholds.