"There is a relationship between autism and seizures; as many as 40 percent of children and young adults with autism may experience seizure, and adolescence is a particular time of vulnerability," said Dr. Bryan King, director of child and adolescent psychiatry at Seattle Children's Hospital's Research Center for Health Services and Behavioral Research.
"There are hormonal changes that could increase the risk of seizure, and certainly there are ongoing brain changes that take place during adolescence, but no one knows why the risk increases in older children."
Speaking for John Travolta, Ossi said, "This is the worst day of his life."
Ossi said that Dr. Mark Smith, the Travolta family doctor, plans to give a statement after the autopsy.
ABC News' Eileen Murphy, Lauren Cox and Kirk Fernandes contributed to this report.