BERLIN, April 11 (Reuters) - German police said on Tuesday they still did not know who was behind what they called a serious explosives attack on the team bus of the Borussia Dortmund soccer team that injured one player.
"At this time, it is still not clear what the real background to this act is," Dortmund police chief Gregor Lange told a news conference.
He said police assumed the team bus was deliberately targetted for the attack as it left the team hotel on the way to the stadium for a Champions League match later on Tuesday.
"The risk of a terrorist attack is not new today," Lange added. "We have been preparing for this for a long time. I do not want to suggest that this was a terrorist attack. All that is still being investigated. We want to be careful. It is being investigated very professionally."
An official for the state prosecutor's office said that a letter was found near the blast site but that it was unclear if the letter was authentic. She declined to provide any details about the contents of the letter.
A spokesman for Borussia Dortmund said the injured player, Marc Bartra, was being operated on for a broken bone in his hand. (Reporting by Paul Carrel and Erik Kirschbaum)