"We lost our girl," Helen McCole Bartusiak told CNN.
While investigators have not determined a cause of death, Bartusiak had been suffering from epileptic seizures in recent days, according to her mother.
"She was a kind and really beautiful girl," her mother said.
Photos: People we lost in 2014
Bartusiak's most visible role was as the young daughter of Mel Gibson's Revolutionary War "Patriot" character in the 2000 movie.
Her movie acting career began when she was just 6 in 1999 with a role in "The Cider House Rules."
She played the daughter of Michael Douglas' character in "Don't Say a Word" in 2001.
Her last film role was the lead in "Sick Boy," a low-budget thriller released in 2012.
Bartusiak had been preparing to produce and direct her first feature film in recent months, her mother said.
Her boyfriend found Bartusiak sitting up in her bed in the garage apartment adjacent to her parents' Houston home, her mother said. "We think she had a seizure and choked and nobody was there."
Her mother said she started cardiopulmonary resuscitation on her daughter before paramedics arrived. "They were working on her for 45 minutes and could not get a heartbeat," she said.
"I've done CPR on that kid more than one time and it just didn't work this time," Bartusiak said.
She's suffered epileptic seizures since she was a baby, although they disappeared for a few years until returning last week, her mother said.
Bartusiak's mother spent Sunday morning looking through photos of her actress daughter for the funeral. The images include pictures with Presidents George W. and George H.W. Bush, Mel Gibson, Dennis Hopper and Michael Douglas, she said.
"The girl has lived such an amazing life," she said.