The Office of the Attorney General of Germany is investigating after three explosions rocked the Borussia Dortmund team bus as it drove to their home stadium for Tuesday's Champions League match, injuring defender Marc Bartra and causing the game to be postponed.
The office usually deals with cases that are considered to be of a serious nature, including those in which terrorism is suspected.
Spokeswoman Frauke Koehler said on Wednesday that prosecutors would make a further statement in Karlsruhe on Wednesday afternoon.
Dortmund later confirmed that Bartra had suffered a broken wrist and was undergoing surgery after windows were shattered in three explosions near the bus.
Police in the city said an officer who was escorting the team bus was also injured in the blasts.
UEFA said the Champions League quarterfinal first leg against Monaco had been postponed until Wednesday at 6:45 p.m. local time (12:45 p.m. ET).
Dortmund originally said in a tweet that an "incident" had occurred as the bus was departing for the Westfalenstadion.
They later confirmed an explosion.
A police statement said: "Initial findings make police believe that there was an attack with serious explosives. Those explosives might have been hidden in a hedge near a parking lot."
Public prosecutor Sandra Lucke said a note claiming responsibility for the incident was found near the site of the explosions. Police were attempting to authenticate the note.
"The background [of the attack] is being investigated thoroughly. The investigations are based on a suspicion of [attempted] homicide," Lucke said.
"A claim of responsibility was found near the crime scene. The authenticity is currently being attested. I can't tell you more about the content because of ongoing investigations. We are investigating in all directions."
Dortmund police president Gregor Lange added: "We have to believe that it was a targeted attack on the BVB squad.
"The decision has been made to play this match tomorrow. We are preparing for a major police operation, and have already initiated safety measures for both teams.
"We will do our best that the match can go through safely tomorrow, but there can never be 100 percent safety. We will do all we can to ensure it."
Dortmund CEO Hans-Joachim Watzke said the team's location after the incident was initially not being shared for security reasons.
He added: "The current situation seems to be this. When the team left the hotel and drove onto Wittbraucker Strasse, apparently three explosives were hidden in a hedge, at least that's what I hear from police sources, and somehow detonated.
"This naturally affected the team bus and Marc Bartra was injured to his hand and arm. He was treated on site and taken to hospital. It is nothing life-threatening or something like that.
"The team is totally shocked, that's clear. Our job now is to cope with this somehow. We must play in under 24 hours. That's our job."
Fans had already begun to enter the stadium, and an announcement said the situation there was "safe" and that there was "no reason to panic."
The fans left the stadium without incident when the game was called off, and Monaco then held a training session on the pitch.
UEFA said in a statement that the decision to postpone the game "was made after a meeting held at the Westfalenstadion between UEFA, representatives of the two clubs and local authorities."
"[Coach] Thomas [Tuchel] was shocked, one of the explosions happened on his side [of the coach]," Watzke said.
"The team is in shock. It will not be easy to get those pictures out of your head. We hope the team is somehow capable to be competitive again tomorrow."
Dortmund told fans to retain their tickets for the rearranged game on Wednesday, and asked travelling Monaco supporters to tweet "#bedforawayfans" if they needed to find lodging for the night.
Monaco said all their fans staying in Dortmund for the night would be reimbursed up to €80 by the club.
UEFA president Aleksander Ceferin said postponing the game was the right move "since we must always prioritise the safety and security of all fans, team officials and players."
FIFA president Gianni Infantino also issued a statement that 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 condemns the incidents in Dortmund. We are all awaiting further details of the incident."
There have been a number of previous incidents involving football teams' buses.
In 2010, three people died when Angolan rebels attacked the Togo national team bus with machine guns on the Congolese border ahead of the African Nations Cup.
In 2015, a bus carrying Fenerbahce players was shot at in northern Turkey, injuring the driver, and later that year Hertha Berlin's team bus was shot at ahead of a German Cup match in Bielefeld. Ther were no reported injuries.
Last September, six armed men robbed a bus carrying Venezuelan first division club Trujillanos. Nobody was hurt.
Information from the Associated Press was used in this report.