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German police have detained an "Islamist" suspect over three explosions that struck Borussia Dortmund's team bus, prosecutors said on Wednesday, confirming that the investigation was examining a possible "terrorist link".

"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," said lead prosecutor Frauke Koehler.

Dortmund lost its rescheduled Champions League quarterfinal with Monaco 3-2 on Wednesday after tributes were paid to the player injured when three explosions targeted the German team's bus.

Amid heightened security, the defeat for Borussia Dortmund in Europe's top club competition came less than 24 hours after the attack rattled nerves across the city in western Germany.

Armed police officers in body armour patrolled the streets around Dortmund's stadium on Wednesday night as locals and visiting fans mingled in a subdued atmosphere.

Supporters were banned from bringing backpacks to the match and some were frisked - with security officials even checking under their hats. During the match, small knots of armed police guarded access to the stands.

Earlier in the day, Koehler said investigators are focusing on two suspected "Islamic extremists" in the bus attack and searched their homes, arresting one of them. But authorities said other motives are possible.

Investigators are still trying to determine how the metal-packed devices were detonated and what explosive substance was used.

They also found three copies of a note at the scene of the blasts, which demanded the withdrawal of German Tornado reconnaissance jets that are assisting the fight against the Islamic State group and the closure of the US Ramstein Air Base in Germany, Koehler said.

But the region's top security official raised the possibility the note could be "an attempt to lay a false trail."

"We are investigating in every direction," said Ralf Jaeger, the interior minister of North Rhine-Westphalia state.