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 one of the three explosives used in Tuesday evening’s attack contained metal strips.
Two letters claiming responsibility for the attack were being investigated, they said.
Prosecutors are treating the blasts as a terror attack but say the precise motive is unclear at present.
Chancellor Angela Merkel on Wednesday said the attack was “an appalling crime” and praised the fans of both teams for coming together.
A spokeswoman for Germany’s federal state prosecutor in Karlsruhe, 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.”
The blast radius of the attack was about 100m and prosecutors said it was lucky the casualties were not worse.
Ms Koehler said a piece of shrapnel had embedded itself in the headrest of one of the seats on the team bus.
She 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.
IS had said it carried out the attack on a Christmas market in Berlin in December that killed 12 people.
Ms Koehler said the letters had demanded “the withdrawal of [German] tornado fighter jets from Syria and, I quote, the closure of Ramstein airbase.”
The text is being analysed to see if it is authentic.
The Sueddeutsche Zeitung newspaper said the letter began with the phrase “in the name of Allah”.
But is said it was possible the perpetrators were deliberately trying to mislead the investigation.
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.