The preliminary report comes on the heels of comments by new interior minister Carlos Basombrio that there is strong evidence police officers ran death squads in to secure promotions and other benefits.
The revelations are fuelling louder calls for an overhaul of Peru’s free-wielding police force, which has been dogged by allegations of corruption and involvement in organised crime under former president Ollanta Humala.
Those killed were originally presented as dangerous outlaws involved in crimes such as bank robbery and kidnapping.
However, deputy interior minister Ruben Vargas said at a news conference that 11 of the 20 killed did not have criminal records and accused officers of falsifying intelligence reports.
Mr Vargas did not name any of the officers under investigation but said one of them was decorated six times.
Media led by La Republica first published allegations this month that up to 90 officers were alleged to have been involved.