Three explosions rocked the Borussia Dortmund bus, injuring Spanish international Marc Bartra, as the German team headed for a Champions League game against Monaco on Tuesday.
Bartra had a wrist surgery after being hit by flying glass and the quarterfinal match was postponed until Wednesday because of the attack. The club said the other Dortmund players were "shocked" but safe.
"We are assuming that they were a targeted attack against the Dortmund team," said the western German city's police chief Gregor Lange, adding however that it did not amount to an organised terror assault.
Local prosecutors also revealed that a letter has been found close to the site of the blasts, which struck shortly after the team bus drove off from the squad's hotel.
"Its authenticity is being verified," prosecutor Sandra Luecke told journalists, without giving details on its contents.
The bus had set off for the Borussia stadium about 10 kilometres (six miles) away when "three explosive charges detonated," said police.
The explosives, which went off shortly after 7.00pm (1700 GMT), were hidden in a hedge and were set off as the bus passed.
The blast shattered the bus windows and the vehicle was burned on the right hand side.
"The bus turned onto the main road, when there was a huge noise a big explosion," Dortmund's Swiss goalkeeper Roman Burki told Swiss media.
"After the bang, we all crouched down in the bus. Anyone who could, threw himself on the floor.
"We did not know if more would come." Burki said Bartra was "hit by splinters of broken glass".
Dortmund's press spokesman confirmed the 26-year-old "will be operated on, he has broken the radius (bone) in his right wrist and there are foreign bodies in the arm".
@ChampionsLeague quarter-final first leg postponed until tomorrow #bvbasm More info: https://t.co/UWIkUT8U2O pic.twitter.com/wk3HPrRPxq
— Borussia Dortmund (@BVB) April 11, 2017
The club said other players were safe and there was no danger inside the Signal Iduna Park stadium.
Star striker Pierre-Emerick Aubameyang was seen being led away from the team bus through the traffic.
"The whole team is in a state of shock, you can't get pictures like that out of your head," Dortmund CEO HansJoachim Watzke said.
"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."