Police investigating the suspected targeted bombing on German football team Borussia Dortmund's team bus have found a letter claiming responsibility.
Detectives gave no further details of the letter in the wake of Tuesday's attack that left one player in hospital and forced the side to postpone their Champions League match against French side Monaco.
Defender Marc Bartra, 26, had an operation for a broken bone in his right wrist after windows shattered in three explosions at around 7.15pm as the squad left their hotel en route to the Westfalenstadion.
An officer who was accompanying the bus on a motorbike suffered blast trauma and shock.
The first leg of the quarter-final tie will now be played on Wednesday.
Police spokeswoman Nina Vogt told ZDF television on Wednesday the probe into the blasts remains "open", adding: "Of course our investigations have to go in all directions to begin with."
In the aftermath of the attack, police chief Gregor Lange said: "We must start from (the position this was) a targeted attack on BVB's team.
"At the present time it is not clear what the background of the act is."
Goalkeeper Roman Burki, who was sat alongside Bartra on the back of the bus, told Swiss newspaper website blick.ch: "The bus turned down the main street, and there was a giant explosion.
"I was sitting in the back row next to Marc Bartra, who was hit by fragments of the broken glass.
"After the bang, we all ducked in the bus and those who could threw themselves to the ground. We did not know had happened.
"The police were quick on the scene, and we were safe. We're all shocked - nobody thought of a football match in this moment."
Dortmund chief executive Hans-Joachim Watzke said: "The whole team is in shock. We have to get through this. It will not be easy for the players. I hope that the team will somehow be able to return to the field.
"Borussia Dortmund is especially strong in extreme situations. Everyone will come even closer together and I'm sure the team will feel this."
Monaco said all their supporters staying in Dortmund on Tuesday night would be reimbursed up to €80 (£68) by the club, while the German club began using the hashtag #bedforawayfans to ask locals to provide accommodation for away fans for the night.
Speaking after the stadium was evacuated, Monaco fans said they had "seen love" from their rival supporters.
But in reference to the explosion, one fan said: "This is not love. This is shame.
"And tomorrow we are coming back to support our team because football is love and football is more powerful than everybody."
Uefa president Aleksander Ceferin said he was "deeply disturbed" by the blasts and backed the decision to reschedule the match.
Fifa president Gianni Infantino condemned the incidents and said the thoughts of "every one of us at Fifa are with the people of Dortmund".
"We are closely monitoring the condition of BVB's Marc Bartra, and wish him a speedy recovery from his injuries," he said.
Bartra, who joined Dortmund from Barcelona last year, was inundated with messages of support, including from his former club.