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An Islamic extremist has been arrested after a bomb attack on the Borussia Dortmund football team's bus.

Dortmund defender Marc Bar tra, 26, and a police officer were injured on Tuesday night following three explosions as the players headed to their stadium for a Champions League quarter-final match against AS Monaco.

German prosecutors said that two Islamic extremists were the focus of the attack probe and that one of them has been detained.

They also said a letter found near the site of the explosions suggests a possible Islamic extremist motive for the attack.

Speaking at a news conference, Frauke Koehler, a spokeswoman for federal prosecutors, confimed the two suspects had become the focus of the investigation.

She said their apartments were being searched and that one of them had been detained.

Ms Koehler said the letter at the attack site demanded 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.

The Süddeutsche Zeitung newspaper in Munich claimed the letter found at the scene claimed responsibility for Islamic State and condemned flights over Syria by German Tornado planes.

It was also said to mention the Christmas market truck attack in Berlin on December 19 in which 12 people died, and said sportsmen and other celebrities “in Germany and other Crusading nations” were “on a death list of the Islamic State” because of the West’s involvement in Syria.

The newspaper also reported that the letter demanded withdrawal of the German Tornados from Syria as well as the closure of the US air base at Ramstein in Germany.

Koehler said authorities have "significant doubts" about another letter posted online claiming an anti-fascist group carried out the attack.

The second letter claimed an anti-fascist group carried out the attack because Dortmund had not done enough to fight racism, right-wing populism and neo-Nazism and said the attack was a “symbol of the politics” of the club.

The official also revealed the explosive devices used in the attack contained metal pins, one of which buried its way into a headrest on the vehicle.

Bartra needed surgery after suffering a broken bone in his right wrist and to remove shrapnel from his body. His fiancée Melissa Jimenez was at his bedside and he was expected to make a full recovery.

The injured policeman was a motorcycle outrider escorting the coach to the stadium when the three bombs detonated. He suffered hearing damage.

The devices went off as the vehicle was six miles from the Westfalenstadion venue.

Coach driver Christian Schulz, 46, managed to keep control of it as the blasts shattered windows and players cried out in panic.

Borussia Dortmund fans opened up their homes to give stranded Monaco fans somewhere to stay for the night.

The football match was cancelled and rescheduled for Wednesday evening.