German investigators have expressed "significant doubts" that Tuesday's attack on the Borussia Dortmund team bus was the work of radical Islamists.

As the team left their hotel ahead of the first leg of their Champions League quarter-final with AS Monaco, three explosions hit the vehicle, with defender Marc Bartra suffering a broken hand in the incident.

Three identical letters printed in German found near the scene of Tuesday's attack in Dortmund claimed that it was carried out "in the name of Allah".

However, a report commissioned by investigators said there were "significant doubts" about the letters and suggested they had been written to trick people into thinking there was an Islamist militant motive.

"That's accurate," Frauke Koehler, a spokeswoman for the federal public prosecutor's office, said when asked to comment on the ARD report. "It is indeed doubtful."

Bild newspaper, meanwhile, quoted an investigator as saying: "The overall circumstances lead us to believe it's most likely that the perpetrators have a right-wing background."

Another German newspaper, Tagesspiegel, said that it had received an email from someone with far-right motives, who said the attack was a warning and made racist threats.

Tuesday's game was postponed to the following day, with Dortmund falling to a 3-2 defeat.