Skye McCole Bartusiak, best known for playing Mel Gibson's young daughter in The Patriot, died of an accidental overdose of painkillers and muscle relaxants, a source told People on Sunday.
Bartusiak, 21, died in her sleep in her Texas home last July.
Though Bartusiak's mother said her daughter suffered from epileptic seizures, authorities described the cause of death as "an accident due to the combined effects of hydrocodone and difluoroethane with carisoprodol."
Hydrocodone, commonly found in Vicodin, is used to treat pain, while carisoprodol is a muscle relaxant. Difluoroethane is a refrigerant that is sometimes used to get high by huffing.
Bartusiak also starred in 2001's Don't Say a Word and 1999's The Cider House Rules.