Skye McCole Bartusiak Died of an Accidental Overdose
10/13/2014 AT 08:25 PM EDT
The child star, who found fame as Mel Gibson's daughter in The Patriot, died in her sleep at her Texas home in July. She was 21.
"The exact cause of death is an accident due to the combined effects of hydrocodone and difluoroethane with carisoprodol," a source at the Harris County Institute of Forensic Sciences tells PEOPLE.
Hydrocodone is found in pain killers, carisoprodol is a muscle relaxant and difluoroethane is an inhalant found in household products that can be used to get high, according to the National Institutes of Health.
Bartusiak's mother, Helen, told CNN that her daughter had been suffering from epileptic seizures before her death.
"She was a kind and really beautiful girl," she said. "This girl has lived such an amazing life."
Reporting by GABRIELLE OLYA