Borussia Dortmund's team bus was attacked with explosives on Tuesday shortly before the start of their Champions League clash with AS Monaco, injuring defender Marc Bartra and forcing the match to be postponed by a day.
German police said on Tuesday they did not know who was behind the attack, in which three explosions went off at 7:15 p.m. near the hotel where the team was staying, but said the team appeared to be the target.
Prosecutors said a letter had been found near the scene of the blast, but declined to give details of its contents and said it was not clear whether it was authentic.
"At this time, it is still not clear what the real background to this act is," Dortmund police chief Gregor Lange told a late night news conference in the western German city.
Police earlier said there had been no risk to the Signal Iduna Park stadium, the largest in Germany, holding more than 80,000 fans, where the first leg of the quarter-final of Europe's top club soccer competition was due to have been played.
Lange said police assumed the team bus was deliberately targeted in the attack as it left the team hotel on the way to the stadium.
The devices were placed in a bush alongside the street, Bild newspaper reported, without giving a source. A police spokesman had earlier said "the explosive devices were placed outside the bus".
Windows on the bus were broken but the damage was limited.
Dortmund police said earlier on Twitter: "After the initial investigation, we assume that this was an attack using serious explosives."
While the motive was unclear, it revived memories of the November 2015 attacks in Paris that targeted entertainment venues including the Stade de France where France were playing Germany in a soccer friendly.
A deadly truck attack at a Berlin Christmas market in December killed 12 people - putting the issue of security at the heart of the national election on Sept. 24, in which Chancellor Angela Merkel is running in a tight battle for a fourth term.
"The risk of a terrorist attack is not new today," Lange said.
"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."
German interior minister Thomas de Maziere said on Twitter: "My thoughts are with the team. It's important now to get to the bottom of this. I hope that football will be the focus again tomorrow."
State prosecutor Sandra Luecke said the letter found near the blast site was being examined. She declined to provide details about the contents of the letter.
"The investigation is at this time is for attempted manslaughter. A letter was found near the scene of the crime. The authenticity of the letter is being examined," she said.
Terrorism expert Davis Lewin told Bild that the explosive devices appeared to have been "quite weak".
"That doesn't really fit in with the strategy of Islamist attacks, which until now have gone for large numbers of victims such as in the (truck) attacks in Nice and Berlin," he said.
"On the other hand, terror organizations like ISIS have called upon their followers to go out with home-made bombs. It wouldn't be the first time that something didn't work out the way it was planned. It's just too early to tell what's behind this attack."
A spokesman for Borussia Dortmund said the injured player, Bartra, was being operated on for a broken bone in his right wrist and shrapnel in his arm.
"The bus turned into the main street, when there was a huge boom, a real explosion," Sky television quoted Dortmund goalkeeper Roman Buerki as saying.
"I was sitting in the back row next to Marc Bartra, hit by fragments ... after the bang, we all ducked."
Bartra, 26, joined Dortmund for 8 million euros ($8.5 million) last year from Barcelona, after coming through the Catalan club's youth system. He has made 12 appearances for the Spanish national team.
Borussia Dortmund's managing director Hans-Joachim Watzke said: "The whole team is in a state of shock."
The stadium emptied quickly and without incident.
AS Monaco goalkeeper Danijel Subasic told Croatian newspaper 24sata: "We are currently in the stadium, in a safe place, but the feeling's horrible."
Dortmund and UEFA said the match would go ahead on Wednesday at 1645 GMT (1245 ET).
UEFA President Aleksander Ceferin said: "I was deeply disturbed by the explosions which occurred tonight in Dortmund."
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."
(Feature image source: Reuters)