Officer Edsaul Mendoza is charged with murder in the March 2022 shooting death of Thomas "TJ" Siderio.
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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.
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Mendoza is still awaiting trial.
There is no word yet on what the lawsuit is seeking.