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 terrorists 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.

Welt am Sonntag said police thought further attacks were possible, citing a document sent to regional police headquarters as saying that soccer games, rock concerts and cultural events were particularly at risk.

It said regional police wanted to have a greater presence if necessary and use dogs trained to sniff out explosives.

Increased Security

Joachim Herrmann, interior minister of the state of Bavaria, told the same newspaper Bavaria planned to use more video surveillance, especially in crime hotspots and in public places - a controversial issue in Germany, where memories of the Nazi Gestapo and the Stasi security police still linger.

Joachim Thomas, chairman of the association of German stadium operators, told Welt am Sonntag he believed full-body scanners would be used at entrances to stadiums in future.

Arnold Plickert, GdP police union's deputy chairman, said police were overstretched already and did not have enough staff to keep watch over a team's hotel night after night so clubs would have to provide extra security staff if they wanted that.

Security was tight for Saturday's matches, including a Bundesliga game between Dortmund and Eintracht Frankfurt.

Dortmund police posted a picture on Twitter showing an armed officer outside the stadium, saying they had provided more security inside and out. Police in the western city of Mainz said they had safely accompanied buses carrying both the Mainz and Hertha BSC soccer teams to their match on Saturday.