German police have detained a suspect with "Islamist links" following a bomb attack on the bus of the Borussia Dortmund football team.
Prosecutors also said the three explosives used contained metal strips.
Two letters claiming responsibility for the attack were being investigated, they said.
Officials are treating the blasts as a terror attack, a spokeswoman for Germany's federal state prosecutor in Karlsruhe said.
Federal prosecutors, who usually lead terror investigations, have now taken over the probe.
Lead prosecutor Frauke Koehler said: "Two suspects from the Islamist spectrum have become the focus of our investigation. Both of their apartments were searched, and one of the two has been detained."
Ms Koehler said three letters containing the same text found near the site of the blasts indicated that the attacker had links to so-called Islamic State.
She said: "Among other things they demand the withdrawal of [German] tornado fighter jets from Syria and, I quote, the closure of Rammstein airbase."
That letter is being analysed to see if it is authentic.
Another letter was published online, in which left-wing extremist groups claimed to have carried out the attack, but the state prosecutor had reason to believe this letter was not authentic.
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