Officer Edsaul Mendoza is charged with murder in the March 2022 shooting death of Thomas "TJ" Siderio.
Mendoza was working undercover when he fatally shot Siderio in South Philadelphia.
According to investigators, Siderio was unarmed and on the ground when he was shot.
Mendoza is still awaiting trial.
There is no word yet on what the lawsuit is seeking.