A suspect with “Islamist links” has been arrested by German police following a bomb attack on the bus of the Borussia Dortmund soccer team.

German prosecutors also said the three explosive devices contained metal pieces.

Two letters claiming the attack on April 11 were being investigated, they said.

Prosecutors are treating the blasts as a terrorist attack, but say the precise motive is unclear at present.

On April 12, German Chancellor Angela Merkel said the attack was “an appalling crime” and praised the fans of both Dortmund and their Champions League opponents, Monaco, for coming together.

Fans later filled Signal Iduna Park in Dortmund for the rescheduled quarter-final first-leg, which kicked off at 18:45 local time.

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There was a rousing rendition of You’ll Never Walk Alone by both sets of fans before kick-off.

Earlier, a spokeswoman for Germany’s federal prosecutor, Frauke Kohler, 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.”

German media are reporting that the suspect detained is a 25-year-old Iraqi, and the second suspect is a 28-year-old German.

The explosion radius of the attack was about 100m. Prosecutors said it was lucky the casualties were not worse.

Frauke Kohler 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 ISIS. The terrorist had said it carried out the attack on a Christmas market in Berlin in December that killed 12 people.

Frauke Kohler said the letter demanded “the withdrawal of [German] tornado fighter jets from Syria and, I quote, the closure of Ramstein airbase.”

Ramstein is a significant US Air Force base. The text is being analyzed to see if it is authentic.

The Sueddeutsche Zeitung newspaper said the letter began with the phrase “in the name of Allah”.

However, the newspaper 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.

Borussia Dortmund players were on their way to their Champions League match, when three explosive charges detonated, police said.

Spain international 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.