GERMAN prosecutors say a letter found near the scene of the Borussia Dortmund team bus blasts suggests a possible Islamic extremist motive for the attack, and one suspect has been taken into custody.

Frauke Koehler, a spokeswoman for federal prosecutors, said that two suspects from the “Islamist spectrum” have become the focus of the Dortmund investigation.

She said at a news conference that both of their apartments were searched and one of them has been detained.

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Koehler said the letter at the attack site demanded the withdrawal of German Tornado reconnaissance jets from Turkey and the closure of the US's Ramstein Air Base in Germany, adding that authorities were still evaluating its credibility.

She said authorities have “significant doubts” about another claim online giving a left-wing extremist motive for the attack.

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.

Koehler told reporters that “we can talk about luck that nothing worse happened”.

She said investigators are still working to determine how the devices were detonated on Tuesday night and what substance was used.

Dortmund player Marc Bartra was injured in the explosions, and has had surgery on this arm and wrist.

The team bus was driving to a Champions League match against Monaco when the blast occurred.

Earlier, Chancellor Angela Merkel’s spokesman condemned the explosions as “a repugnant act” and said the German leader has spoken with the club’s chief executive.

Spokesman Steffen Seibert told reporters that Merkel was “dismayed” by the attack.

He said she spoke by phone with Dortmund’s Hans-Joachim Watzke and wished the team “all the best”.

Interior Minister Thomas de Maiziere attended the rescheduled Champions League quarter-final match last night.

Ministry spokesman Tobias Plate said that he was travelling to Dortmund as a sign of solidarity.