German police are investigating a possible Islamist link to three explosions that rocked the Borussia Dortmund football team's bus, after a letter found at the scene referred to the devastating Berlin Christmas market attack, national media reported Wednesday.

The letter "claims responsibility for what happened," prosecutor Sandra Luecke had said late Tuesday, telling journalists that "its authenticity is being verified."

Luecke did not give details of its content, but national media citing unnamed sources said it referred to the Berlin attack in December claimed by the Islamic State group that killed 12 people.

It also mentioned Germany's deployment of Tornado reconnaissance missions as part of an anti-IS international coalition, they reported.

The assault that wounded two, described by Dortmund city's police chief as a "targeted attack" against the team, shook German football ahead of crucial Champions League ties.

The Dortmund team had been leaving for the stadium to face Monaco when the blasts occurred.

Their match is now delayed until later Wednesday, when Bayern Munich are due to host Real Madrid for a separate Champions League game.

The games will take place under heightened security, with hotels of the players under guard and their buses driven to safe locations.

German authorities have until now held off from describing it as a terror attack, saying it is too early to determine a motive.

But the probe has now been taken over by federal prosecutors, whose remit includes terror investigations.

Germany has been on high alert since a series of jihadist attacks last year, including the Christmas market truck 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.

Spanish international Marc Bartra underwent surgery on a broken wrist after he was hit by flying glass. The injured policeman, who was on a motorcycle and had been 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.

- Shocked players -

The explosives, which went off shortly after 7:00 pm (1700 GMT), were hidden in a hedge and were detonated as the bus passed.

The blast shattered the bus windows and the vehicle was burned on the right hand 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."

Burki said Bartra was "hit by splinters of broken glass". Dortmund's press spokesman said the 26-year-old had broken the radius bone in his right wrist.

The club said other players were safe and there was no danger inside the Signal Iduna Park stadium.

"The whole team is in a state of shock, you can't get pictures like that out of your head," Dortmund CEO Hans-Joachim 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."

- 'Hard to absorb' -

Germany's best-selling Bild daily quoted anonymous sources saying that investigators were hunting for a likely getaway car used by the attacker.

The vehicle had foreign car plates, said the newspaper, which also 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 Tuesday's match was due to start.

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".

Dortmund's president Reinhard Rauball said he believed the team would be ready for Wednesday's game.

"The players will be able to push this out of their minds and be in a position to put in their usual performances," he said.

"The worst thing would be if whoever committed this attack was now able to get to affect them through it."

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."

Source: AFP