Investigators found near the scene a handwritten letter that claimed responsibility for the attack, prosecutor Sandra Luecke said. The attack was claimed by the Islamic State group.
The letter referred to the Berlin Christmas market attack in December claimed by the Islamic State extremists group that killed 12 people, as well as Germany's deployment of Tornado reconnaissance missions as part of an anti-ISIS global coalition.
Borussia Dortmund players have taken to their own cars to get to and from a training session the morning after explosives damaged their team bus as it drove toward their Champions League semifinal against Monaco.
Mr Watzke said: "It was all very disciplined".
"We will play not only for ourselves today".
A police officer, who had been travelling on a motorbike in front of the bus, was also injured.
Terrorism expert Davis Lewin told Bild that the explosive devices appeared to have been "quite weak".
Federal prosecutors usually take on cases that are considered to be of a serious nature, including those in which a terrorist motivation is suspected. "The authenticity of the letter is being examined", she said.
Germany has been on high alert since a series of jihadist attacks previous year, including the Berlin market assault.
Security is being reviewed at all three of Wednesday's Champions Leagues games, the day after Borussia Dortmund's team bus was targeted by a bomb attack traveling to its home stadium.
Dortmund defender Marc Bartra needed surgery on injuries to his wrist and arm.
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Police said they believe the explosions were a "targeted attack" against the team and that the devices may have been hidden in a hedge near a parking area.
The explosives shattered the bus windows and the vehicle was burned on the right hand side.
Goalkeeper Roman Burki was sitting at the back of the team bus alongside Bartra, and told Swiss newspaper website blick.ch how events unfolded.
"After the bang, we all ducked in the bus and who could, put on the ground". We did not know had happened.
His father said Bartra is shocked by the attack.
Police were unable to specify what type of explosives were used but they described the devices as "serious", stating they believed they had been hidden under a hedge next to a auto park.
The announcement that the game was postponed was only made to the stunned stadium about 15 minutes before kick-off on Tuesday.
Dortmund have also launched a campaign for supporters to provide accommodation for Monaco fans left stranded in the city.
German investigators are looking into the possible involvement of Islamist militants or anti-Nazi activists in blasts that hit a bus carrying players from soccer club Borussia Dortmund, media reported on Wednesday. "If the players are able to shake this off is an entirely different matter".
However, another police spokeswoman, Nina Vogt, said there was no further information yet on what caused the explosions.
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