German authorities have arrested a suspected Islamic extremist connected to what Chancellor Angela Merkel called a "despicable" attack.
Ms Merkel said she was appalled by this week's attack, in which three explosions went off in a bus carrying players of one of the country's top soccer teams.
According to police, an explosion detonated as the bus was leaving the hotel where the team was staying to bring them to their Champions League game against Monaco.
The explosions occurred as the Borussia Dortmund bus made its way to a stadium match, and blasts smashed windows on the bus transporting players, including Spanish defender Marc Bartra, who suffered injuries to his arm and wrist.
A team spokesman said Mr Bartra was operated on for a broken bone in his right wrist and shrapnel in his arm.
Ms Merkel praised Dortmund fans for offering accommodation to fans after the match was postponed and said, "We all agree that we are dealing with a despicable act".
The attack forced a 24-hour postponement of the team's high-profile clash with Monaco, which turned out to be a riveting encounter that the visitors won 3-2.
Borussia Dortmund coach Thomas Tuchel said after the game that while he encouraged everyone to take the game seriously, "football is not the most important thing in the world".
An exit from the Dortmund stadium was briefly closed as police examined suspicious packages, but quickly re-opened.
Ralf Jaeger, interior minister in the state encompassing Dortmund, said the investigation was looking "in all directions", and it was unclear if one or several attackers were involved.
Frauke Koehler, spokeswoman for the Federal Prosecutor's Office, which handles probes into suspected terrorism, said the explosive devices had contained strips of metal.
Ms Koehler said investigators had found three letters near the scene, all with the same content suggesting a possible Islamic extremist motive.
The letters referred to the use of Tornado reconnaissance planes in Syria, she said, which Germany had deployed as part of the military campaign against Islamic State, and also called for the closure of the US military base at Ramstein in western Germany.
Ms Koehler said investigators had identified two suspects from the "Islamist scene", searched their apartments and detained one man.