German investigators are looking into the possible involvement of Islamist militants or anti-Nazi activists in blasts that hit a bus carrying players from soccer club Borussia Dortmund, media reported on Wednesday.

Sueddeutsche Zeitung newspaper said a letter left near the scene of the attack on Tuesday claimed it was in retaliation for German military reconnaissance missions against Islamic State in Syria. The paper also said the letter might be a deliberate attempt to mislead investigators.

German press agency dpa said investigators were examining a second letter, posted on an anti-fascism online portal, which said the attack was in retaliation for what it called the club's soft approach toward neo-Nazi and racist fans.

The Federal Public Prosecutor said in a statement it had taken over the investigation and would hold a news conference at 2 p.m. (1200 GMT).

Windows on the bus were broken in the attack, in which three explosions went off at 7:15 p.m. on Tuesday near the hotel where the team was staying.

Spanish defender Marc Bartra was injured and the Champions League clash with AS Monaco was postponed by a day until Wednesday. Bartra was operated on for a broken bone in his right wrist and shrapnel in his arm, a team spokesman said.

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.

The blasts revived memories of Islamist militant attacks in Paris in November 2015 whose targets included a stadium where France were playing Germany in a soccer friendly.

Security is among the issues at the heart of Germany's parliamentary election on Sept. 24, a tight battle in which Chancellor Angela Merkel is running for a fourth term. In December, a Tunisian man killed 12 people when he ploughed a truck through a Berlin Christmas market.