German prosecutors say a letter found near the scene of the Dortmund team bus blasts suggests a possible Islamic extremist motive for the attack, and one suspect has been taken into custody.

Frauke Koehler, a spokeswoman for federal prosecutors, said Wednesday that two suspects from the "Islamist spectrum" have become the focus of the Dortmund investigation. She says at a news conference that both of their apartments were searched and one of them has been detained.

She says the letter at the attack site demands the withdrawal of German Tornado reconnaissance jets from Turkey and the closure of the United States' Ramstein Air Base in Germany. She said authorities are still evaluating its credibility.

Koehler says authorities have "significant doubts" about another claim online giving a left-wing extremist motive for the attack. (AP)