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 references the Berlin Christmas market attack and military operations in Syria.
It is not yet clear if the letter is genuine.
Player Marc Bartra underwent an operation after breaking a bone in his wrist. No other players were hurt.
German newspaper Suddeutsche Zeitung said the letter, beginning with the phrase “in the name of Allah”, 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, Suddeutsche Zeitung reported, adding that an analysis of the letter by experts is under way.
Germany’s federal prosecutor, which usually assumes responsibility for terror-related crime, has taken over the investigation.
Borussia Dortmund were on their way to their home Champions League quarter-final match against Monaco, when “three explosive charges had detonated”, police said.
The incident was at Hoechsten, outside the city, at about 19:00 local time.
The first indications were that this was an “attack with serious explosives”, they said.
Pictures from the scene showed the bus’s windows broken and tyres burst in the blasts.
In a news conference, the head of Dortmund police said it was a targeted attack on the team. 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 any more, he said.
Along with player Marc Bartra, a police officer on a motorbike escorting the bus was also wounded.
“Marc Bartra underwent an operation on Tuesday night after breaking the radial bone in his arm and getting bits of debris lodged in his hand as a result of the bomb attack,” his team said in a brief statement.
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 and police thanked the fans for their co-operation.
Despite the apparent claim of an Islamic motive, the attack does not have much in common with previous incidents of that type, the BBC’s correspondent in Berlin, Damien McGuinness, said.
The explosives were not designed to cause maximum damage in a crowd, as in other attacks – or to target the stadium itself, which is several kilometres away from the site.
“One possibility could be that it was a German right-wing extremist attack,” he said.
“This team, Borussia Dortmund, has been plagued recently with violent, far-right hooliganism. A recent clampdown on far right extremists has led to stadium bans, which then resulted to death threats in February for one manager,” he adds.
The letter found at the scene is being authenticated, and police say it claims responsibility – but have not yet released details about its contents.
The match was postponed until 18:45 local time (16:45 GMT) on Wednesday.
“As many officers as possible” would be deployed for tomorrow’s rescheduled game, police said.
Uefa president Aleksander Ceferin said he was “deeply disturbed by the explosions”.
“The decision taken to postpone the UEFA Champions League match between Borussia Dortmund and AS Monaco was the correct one since we must always prioritise the safety and security of all fans, team officials and players,” he said.
Media captionA journalist for German news publisher Bild Online tells the BBC of shock in the stadium
Borussia Dortmund said the attitude within the team was “to play the match tomorrow for Marc”.
The team’s boss earlier said: “The whole team is in a state of shock, you can’t get pictures like that out of your head.
“I hope the team will be in a position to be able to compete tomorrow on the pitch. In a crisis situation like this, Borussia pulls together.
Monaco fans at the stadium were praised for their chants of support for Dortmund after news of the attack emerged.
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.”
Spanish Prime Minister Mariano Rajoy also tweeted his support, wishing Bartra a quick recovery.
Fifa president Gianni Infantino said the football organising body condemned the incident and wished Bartra a “speedy recovery”.
One of the Dortmund players, Mathias Ginter, was involved in the Germany-France match that was targeted in the terror attacks in Paris on 13 November, 2015.
Another present in Paris, Andre Schurrle, was injured for the Monaco match.