Esteban Nunez and three others pleaded guilty to various charges in the killing of 22-year-old Luis Santos.
Santos's parents filed a wrongful death lawsuit against them in September but only went public with it now.
They are seeking unspecified damages.
Just last week, another lawsuit was filed on behalf of the Santos family against ex-Governor Schwarzenegger, who commuted the sentence of Esteban Nunez right before leaving office.
