Borussia Dortmund's team bus was seconds away from sustaining catastrophic damage during last week's attack, an investigator has said in Bild.

Six days after three explosions near the team bus injured two people, including BVB defender Marc Bartra, a 100-strong Bundeskriminalamt (Federal Criminal Police Office) unit continues to follow several leads.

On Sunday, German tabloid Bild reported that the attack could have caused major harm to the Dortmund team.

"If the bombs had been fired around one second earlier, the bus would have taken the full load. Then there would have certainly been several severely injured and maybe even dead people," one investigator said.

The paper added that the investigators are following several leads, ranging from those with Islamist background, far-left and far-right backgrounds to the involvement of hooligans or even the betting mafia.

Although three identical letters found near the scene of the attack suggested a possible Islamist extremist motive, investigators have doubts about their veracity.

One unnamed investigator in Welt am Sonntag claims the explosives used in last Tuesday's attack could "possibly be from German army stocks."

He was quoted by the paper as saying: "The explosive in the pipe bombs which were armed with metal pins possibly originated from German army stocks."

Last Thursday, the Federal Prosecutor's office in Germany said in a statement that one 26-year-old Iraqi detained following the attacks could not be linked with the incident near Dortmund's team hotel. He, however, remained in custody for strong links with the Islamic State for which he allegedly fought in 2014 and 2015.

With investigations still ongoing, Peter Neumann, a German terror expert, warned that there might be new attacks.

"There is no evidence for any theory so far and the real and very dangerous perpetrators remain on the loose," Neumann said. "The top priority has to be to find those responsible, because indeed it cannot be ruled out that there will be more attacks."

After recording a 3-1 Bundesliga win against Eintracht Frankfurt on Saturday, Dortmund are currently preparing for Wednesday's Champions League quarterfinal second leg at Monaco.