German prosecutors say a letter found near the scene of the Dortmund team bus blasts suggests a possible Islamic extremist motive for the attack, and one suspect has been taken into custody.
Spokeswoman Frauke Koehler said the letters referred to the use of Tornado reconnaissance planes in Syria, which Germany has 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.
Frauke Koehler, a spokeswoman for federal prosecutors, said that two suspects from the "Islamist spectrum" have become the focus of the Dortmund investigation.
"It is to be assumed that there is a terror background", a Federal Prosecutor's office statement said.
Dortmund's Spanish worldwide Marc Bartra and a policeman were injured in the roadside blasts set off as the team headed to a Champions League game against Monaco on Tuesday night.
The explosives detonated minutes after the Dortmund team bus pulled away from the squad's hotel and headed for their quarter-final, first-leg tie against Monaco.
The match was called off shortly before kickoff and rescheduled for Wednesday evening.
She also noted an online claim of responsibility by an anti-fascist group, but said there was serious doubt about its validity.
"We are investigating in every direction, and it's really meant that way", said Ralf Jaeger, the interior minister of North Rhine-Westphalia state. "It could be the violent fan scene, it could be Islamist extremism".
Chancellor Angela Merkel condemned the attack as "a repugnant act" and praised the "great solidarity" shown by both teams' fans on Tuesday.
"However, we expect and I am confident that the team will do its best and deliver a spectacle in the Champions League this evening", he said.
In response, UEFA has moved to reconsider safety arrangements for the three games taking place on Wednesday, with Atletico Madrid hosting Leicester City, Real Madrid heading to Bayern Munich, and Dortmund welcoming Monaco for their rearranged meeting.
"We kindly ask all supporters to allow extra time for the possibility of enhanced security checks". Bild also put out a full-page advert in Dortmund's yellow and its BVB 09 logo, with the message: "You'll never walk alone".
Annika Lentwojt, a 21-year-old engineering student, said she was in the stadium Tuesday when the match was called off but "always felt safe".
Professor Atran said they hit "soft targets" that can not be defended in any consistent way "without a tremendous, unaffordable, outlay of resources".
"The precise motive for the attack is unclear at present", it said, adding that police were still analyzing three identical letters discovered at the scene of the attack.
Dortmund is one of Germany's most popular soccer teams and a regular contender for the Bundesliga title, which it last won in 2011 and 2012. The assault was described by Dortmund city's police chief as a "targeted attack" against the team.