A letter found at the scene of the attack "claims responsibility for what happened".
PIC: The damaged bus of Borussia Dortmund is pictured after Tuesday's explosion. (AFP)
German police investigated a possible Islamist link to three explosions that rocked the Borussia Dortmund football team bus as the club vowed Wednesday it won't give in to "terror".
Dortmund's Spanish international Marc Bartra and a policeman were injured in the roadside blasts set off as the team headed to a Champions League game against Monaco on Tuesday night.
The match was put back to Wednesday amid a ratcheting up of security around Dortmund and in Munich where Bayern Munich take on Real Madrid.
Extra forces were deployed around team hotels and their buses will take designated safe routes to the stadiums. UEFA said "security procedures will be enhanced accordingly wherever needed".
A letter found at the scene of the attack "claims responsibility for what happened," prosecutor Sandra Luecke said late Tuesday. The "authenticity is being verified," she added.
Luecke did not give details, but a copy of the letter circulated by national media showed it referring to the Berlin Christmas market attack in December that killed 12 people. The attack was claimed by the Islamic State group.
It also demanded that Germany withdraw its deployment of Tornado reconnaissance missions in the anti-IS international coalition and close the US base at the western German town of Ramstein.
The assault was described by Dortmund city's police chief as a "targeted attack" against the team.
But Dortmund's chief executive Hans-Joachim Watze vowed that his side "will not give in to terror", as Dortmund players returned to training a day after the blasts.
"We will play not only for ourselves today. We will play for everyone... we want to show that terror and hate can never determine our actions," he said in a statement.
Monaco's vice president Vadim Vasilyev said "football must not be taken hostage", as he pledged that the quarter final will go ahead on Wednesday.
German authorities have not called the attack organised terrorism. But the probe has been taken over by federal prosecutors, whose remit includes terror investigations.
Separately, national news agency DPA said a second claim of responsibility emerged online, this time possibly linked to "anti-fascist" far-left groups. But doubts over its authenticity were raised by sources in the radical-left.
Germany has been on high alert since a series of jihadist attacks last year, including the Berlin market assault.
The explosives detonated minutes after the Dortmund team bus pulled away from the squad's hotel and headed for their quarter-final, first-leg tie against Monaco.
Bartra underwent surgery on a broken wrist after he was hit by flying glass, Dortmund president Reinhard Rauball told NTV news channel.
The injured policeman, who was on a motorcycle escorting the team bus, suffered trauma from the noise of the blasts.
"We are assuming that they were a targeted attack against the Dortmund team," said the western German city's police chief Gregor Lange.
The explosives shattered the bus windows and the vehicle was burned on one side.
"The bus turned on to 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. We did not know if more would come." Some players hurled themselves to the ground, he said, adding that Bartra was "hit by splinters of broken glass".
'Hard to absorb'
Germany's best-selling Bild daily quoted anonymous sources saying that investigators were hunting for a getaway car used by the attacker.
The vehicle had foreign car plates, said the newspaper, which added that police believed the explosives were a particular type of pipe-bomb.
The announcement that the game was postponed was only made to the stunned stadium about 15 minutes before kick-off.
In a show of solidarity, some Dortmund fans took in stranded Monaco supporters for the night.
Bild also put out a full-page advert in Dortmund's yellow and its BVB 09 logo, with the message: "You'll never walk alone".
Rauball said he believed the team would be ready for Wednesday's game.
"The worst thing would be if whoever committed this attack was now able to get to affect them through it," he said.
But ex-Dortmund player Steffen Freund, who won the Champions League with Borussia in 1997, said there would be scars.
"When there has been a direct attack on the team bus, then it's not just forgotten by Wednesday," said the 47-year-old.
"Mentally and psychologically that is hard to absorb, it's a lot to deal with."