Reports claim authorities in Hamburg abused power by using a banned weapon against those rioting during the G20 summit in July.
German news website Spiegel claims police shot rubber bullets from a MZP 1 multipurpose pistol towards those rioting at least 15 times.
Authorities in Hamburg claim this is a pistol officers are able to use for domestic security, however Spiegel suggests it has seen a finance ministry document showing the weapon is banned.
Heckler and Koch, which manufactures the pistol, told Spiegel that the gun is classed as a grenade launcher rather than a pistol.
Legal expert Ulrich Karpen from Hamburg University also claims the weapon is not on a list of permitted police weapons, according to The Local.
Spiegel also said police in Hamburg had originally decided officers would not fire rubber bullets towards any protestors but this was changed when violence on streets increased.
Political party Die Linke has announced that it may take legal action against police commander Hartmut Dudde over the matter.
Riots in Hamburg went on for three days where those involved burned out cars and vandalised shops.
Around 500 police officers were injured in the riots, but later at least 35 investigations were opened mainly about assaults on the public by police.