The newspaper "Welt am Sonntag" cited a source involved in the investigation as saying supplies belonging to the German military may have been used to make the explosives used in last Tuesday's attack.

"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," the paper reported on Saturday.

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

Prepare for more attacks

"Welt am Sonntag" said police believe further attacks are possible and that soccer games, rock concerts and cultural events are particularly at risk.

The German Federal Criminal Police Office (BKA) would not confirm the latest report, saying that all lines of inquiry are being examined regarding the attack, which occurred as the bus left the team's hotel for a Champions League match against Monaco. The match was postponed for 24 hours.

New email emerges

The latest revelations came hours after the Berlin-based "Tagesspiegel" newspaper said it had received an anonymous note claiming responsibility for the bombing, which left Dortmund defender Marc Bartra and a police officer injured.

The authors of the email described the bus attack as a "final warning" and threatened another strike on April 22 in Cologne, the day protesters are set to rally against the anti-immigration Alternative for Deutschland (AfD) party.

German federal prosecutors are examining the latest note.

Several leads

The email follows a similar claim of responsibility at the blast site, which investigators said appeared to suggest links to Muslim extremists.

Hours after the bombing, police found three identical letters that allegedly mentioned the "Islamic State" (IS) militia, and demanded that Germany pull its jets from the anti-IS coalition in Syria.

But German public broadcasters and the "Süddeutsche Zeitung" newspaper reported on Friday that investigators had "significant doubts" about the link to militant Islam.

A left-wing group has also claimed it carried out the bombing.

Former Barcelona defender Bartra, who had to undergo a wrist operation, left hospital on Saturday, but will be out of action for about four weeks.

Security was stepped up for Bundesliga matches on Saturday, in which Dortmund beat Frankfurt 3-1 at home.