The explosives used in the attack on the bus of soccer team Borussia Dortmund may have come from supplies belonging to the German armed forces, a newspaper cited a source involved in the investigation as saying on Saturday.

It is still unclear who carried out Tuesday’s attack. German prosecutors have doubts about the authenticity of letters that suggested Islamist militants were behind it and Bild newspaper cited an investigator as saying right-wing extremists were probably responsible.

“The explosives in the pipe bombs, which were filled with metal pins, might have come from the stocks of the German armed forces but that’s still being checked,” newspaper Welt am Sonntag cited a source involved in the investigation as saying.

The source also said that specialist knowledge was required to use the military detonators, which are not easy to get.

A spokeswoman for the federal public prosecutor’s office declined to comment on the report.

The players’ bus was heading to their stadium for a Champions League match against AS Monaco on Tuesday when three explosions occurred, injuring Spanish defender Marc Bartra and delaying the match by a day. Dortmund said on Twitter on Saturday Bartra had been released from hospital.

Read the whole story from Reuters.