(CNN) -- The family of Kendrick Johnson has sued a south Georgia school district claiming it was negligent in protecting their son from harassment and bullying, which they contend led to his death.
Johnson's body was found in a rolled-up mat in the Lowndes High School gymnasium on January 11, 2013.
The lawsuit was filed in State Superior Court on Tuesday. It alleges that Lowndes County Board of Education was negligent and violated Johnson's constitutional right to equal protection based on race. Johnson was African-American.
Investigators with the Sheriff's Office ruled his death accidental -- concluding that Johnson climbed into the center of the gym mat to reach for a shoe and got stuck.
Kenneth and Jacquelyn Johnson believe that their son was beaten to death. They hired an independent pathologist, who found "unexplained apparent nonaccidental blunt force trauma" to the teen's neck and concluded the death was a homicide.
The U.S. Attorney for the Middle District of Georgia launched a federal investigation into the case on October 31, 2013.
An attorney for Lowndes County Schools had not returned CNN's request for comment by Tuesday evening.