Borussia Dortmund attack: Police investigate Islamist link
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.
What happened?
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.
Source: bbc.com