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 explosive devices contained metal strips.
Two letters claiming the attack on Tuesday evening were being investigated, they said.
Prosecutors are treating the blasts as a terrorist 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 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.”
The blast radius of the attack was about 100m. 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 copies of the same letter were found near the site of the blasts, indicating that the attacker had links to so-called Islamic State (IS). IS had said it carried out the attack on a Christmas market in Berlin in December that killed 12 people.
Ms Koehler said the letter 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 it said it was possible the perpetrators were deliberately trying to mislead the investigation.
A second letter was published online, in which left-wing extremist groups claimed to have carried out the attack, but prosecutors had reason to believe this letter was not authentic.
What happened on Tuesday evening?
Borussia Dortmund players were on their way to their home Champions League quarter-final first-leg match against Monaco, when three explosive charges detonated, police said.
Player Marc Bartra underwent an operation after breaking a bone in his wrist. No other players were hurt, but a police officer on a motorbike escorting the bus suffered trauma from the noise of the explosions.
Several reports said the explosives had been hidden in a hedge.
Dortmund goalkeeper Roman Burki told Swiss news outlet Blick that the bus had turned on to the main road when there was a loud noise.
The players ducked to the floor of the bus, not knowing if there would be more blasts, he said.
Captain Marcel Schmelzer added “we’re all in shock” but their thoughts were with their injured colleague.
The 80,000-capacity Signal Iduna Park was later evacuated safely and the match postponed until 18:45 local time (16:45 GMT) on Wednesday.
Who could be behind the attack?
Despite the apparent claim of an Islamist motive, the attack does not have much in common with previous such attacks, says the BBC’s correspondent in Berlin, Damien McGuinness.
The explosives were not designed to cause maximum damage in a crowd – or to target the stadium itself, which is several kilometres away.
Either left-wing or right-wing extremists could also be to blame.
How have people reacted?
Monaco fans at the stadium were praised for their chants of support for Dortmund.
Social media also carried offers from Dortmund residents to Monaco fans in need of a bed for the night on #bedforawayfans.
Fifa president Gianni Infantino said the football organising body condemned the incident and wished Bartra a “speedy recovery”.