German police are investigating a possible Islamic extremist link to the bombing of the Borussia Dortmund football team's bus, German media say.
A letter found near the scene cites the Berlin Christmas market attack and military operations in Syria. It is not yet clear if the letter is genuine.
Meanwhile German federal prosecutors, who usually lead terror investigations, are taking over the probe.
Chancellor Angela Merkel called the bombing a "revolting act".
"The chancellor was, like the people of Dortmund and millions of people everywhere, appalled by the attack on the BVB team bus," her spokesman, Steffen Seibert, said on Wednesday.
The Sueddeutsche Zeitung newspaper said the letter found nearby began with the phrase "in the name of Allah", and mentioned Germany's use of Tornado jets in the coalition forces fighting so-called Islamic State (IS).
IS said it carried out the attack on a Christmas market in Berlin.
But it is possible the perpetrators are deliberately trying to mislead the investigation, Sueddeutsche Zeitung added, saying that an analysis of the letter by experts was under way.
A second letter was circulating online, claiming an anti-fascist group had carried out the attack, Germany's DPA news agency reported.
The second letter claimed the attack was motivated by the club's alleged tolerance for racists and right-wing fans. The club has clamped down on such fans in the past.
Borussia Dortmund players were on their way to their home Champions League quarter-final match against Monaco, when three explosive charges detonated, police said.
Pictures from the scene showed the bus's windows broken and tyres burst.
Player 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 was also wounded.
The head of Dortmund police said it was a targeted attack. 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.
Captain Marcel Schmelzer added "we're all in shock" but their thoughts were with their injured colleague.
Fans already at the 80,000-capacity Signal Iduna Park were told to stay there until it was safe to leave. The stadium was later evacuated safely.
Who could be behind the attack?
Despite the apparent claim of an Islamist motive, the attack does not have much in common with previous such attacks, says the BBC's correspondent in Berlin, Damien McGuinness.
The explosives were not designed to cause maximum damage in a crowd - or to target the stadium itself, which is several kilometres away.
Our correspondent says it could be an attack by right-wing extremists. Borussia Dortmund has been plagued recently with violent hooliganism.
A recent clampdown led to stadium bans, which then resulted in death threats in February for one manager, he says.
Those issues were reflected in the second letter, circulated online through an anti-fascist portal, which referenced the club's allegedly soft policies on "racists, Nazis, and right-wing populists".
What about the match?
The match was postponed until 18:45 local time (16:45 GMT) on Wednesday.
"As many officers as possible" would be deployed for the rescheduled game, police said.
Uefa president Aleksander Ceferin said he was "deeply disturbed by the explosions"
The decision to postpone "was the correct one since we must always prioritise the safety and security of all fans, team officials and players", he said.
Borussia Dortmund said the attitude within the team was "to play the match tomorrow for Marc".
How have people reacted?
Monaco fans at the stadium were praised for their chants of support for Dortmund.
Social media also carried offers from Dortmund residents to Monaco fans in need of a bed for the night on #bedforawayfans.
FC Barcelona, Bartra's former club, tweeted: "All of our support to @MarcBartra, @BVB and their fans."
Fifa president Gianni Infantino said the football organising body condemned the incident and wished Bartra a "speedy recovery".