German police say a letter has been found near the site of a "targeted attack" on Borussia Dortmund's team bus.
Three explosions went off as the bus departed for the stadium for Tuesday night's Champions League match with Monaco and defender Marc Bartra required an operation for a broken bone in his right wrist after he was injured in the incident.
The events unfolded as the squad left their hotel en route to the Signal Iduna Park stadium for the first leg of a quarter-final tie against Monaco, which was swiftly rescheduled for Wednesday.
The local police force - Polizei Nordrhein-Westfalen Dortmund - held a press conference on Tuesday evening along with the football club and the Dortmund public prosecutor.
Police chief Gregor Lange said: "This evening at 19:15 [local time] there were three explosions in the surroundings of the BVB hotel.
"We must start from [the position this was] a targeted attack on BVB's team. Strong police forces are in operation.
"We also have explosive detection dogs in use and decided to search the scene from the air.
"At the present time, it is not clear what the background of the act is."
The authenticity of the letter is being investigated and its contents were not disclosed.
The police force posted on Twitter: "The prosecutor's office #Dortmund confirms the discovery of a letter near the scene of the crime.
"The authenticity is currently being tested."
FIFA president Gianni Infantino said: "The thoughts of every one of us at FIFA are with the people of Dortmund, and the fans of both Borussia Dortmund and Monaco following today's troubling events.
"We are closely monitoring the condition of BVB's Marc Bartra, and wish him a speedy recovery from his injuries.
"FIFA condemn the incidents in Dortmund. We are all awaiting further details of the incident."