An "Islamist" suspect has been detained by police investigating a bomb attack on the Borussia Dortmund team bus.
Two letters left at the scene of the blasts reportedly linked it to German involvement in the fight against Islamic State in Syria.
One mentioned the Berlin Christmas market attack carried out by an IS-inspired terrorist in 2016, according to the Sueddeutsche Zeitung newspaper.
The letter, reportedly written in German, began: "In the Name of Allah, Most Gracious, Most Merciful."
It also said sports stars and other prominent people "in Germany and other crusader nations" were on a "death list of the Islamic State".
The newspaper, quoting security sources, said detectives were looking into the possibility that Tuesday night's attack may have been carried out by Islamist militants.
It also said it might be a deliberate attempt by the attackers to mislead detectives.
The letter claiming responsibility for the blasts was found near the team coach and its authenticity is being verified, according to German federal prosecutors who are spearheading the investigation.
Police are also examining the authenticity of a second letter, according to German press agency dpa, which could have come from anti-fascists apparently claiming that the football club's soft approach towards neo-Nazi and racist fans was the motive behind the attack.
Details of the letters emerged as Dortmund players, including former Chelsea midfielder Andre Schuerrle, returned to training a day after the blasts, with the club's chief executive, Hans-Joachim Watze, tweeting to say the team will "not give in to terror".
An image of the bus after the blast on Tuesday night. Image: Martin Meissner/AP/Press Association Images
Officers are investigating three "serious explosive devices" which went off as the players and coaching staff left the L'Arrivee Hotel and Spa on the outskirts of Dortmund ahead of a Champions League match.
They believe the explosives may have been hidden in a hedge near the car park.
The direction of the investigation remains "open", police spokeswoman Nina Vogt told ZDF television.
The blasts happened around six miles from the Westfalenstadion, where the German team had been due to face French side Monaco in a quarter-final first-leg tie.
Defender Marc Bartra, a Spain international, underwent surgery for a broken bone in his hand after being injured by shattered glass from the coach's windows.
Ralf Jaeger, the interior minister for North Rhine-Westphalia state, said security had been stepped up in Dortmund, including at the club training ground and stadium.
"We might assume that further attacks are planned and we need to take these things seriously," he said.
Monaco fans were offered a place to stay for the night by Borussia Dortmund supporters on social media after the match was postponed for 24 hours.
The German club sent out a tweet about the scheme under the hashtag #bedforawayfans.
Fans of both clubs posted pictures of themselves enjoying meals and a drink together.
Monaco also offered its fans €80 each to stay in a hotel for the night.
Borussia Dortmund president Reinhard Rauball said he was confident the team would rise to the occasion when they do take to the field, despite the ordeal.
"How is the team doing? You can imagine yourself whether they slept well or badly," he said.
"This is certainly a difficult situation for the team which, I think, has not experienced such a situation before.
"However, we expect and I am confident that the team will do its best and deliver a spectacle in the Champions League this evening."
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