Saoirse Kennedy Hill, a granddaughter of Robert F. Kennedy, died of an accidental overdose resulting from a toxic mixture of methadone, alcohol and other prescription drugs, according to a newly issued death certificate.

Ms. Hill, 22, the daughter of Courtney Kennedy Hill, died at the family’s compound in Hyannis Port, Mass., on Aug. 1. Emergency responders were called to the compound in the afternoon and Ms. Hill was taken to Cape Cod Hospital in Hyannis, where she was pronounced dead at 3:14 p.m., according to the death certificate.

Her death added to a litany of tragedies that have befallen the Kennedy family, including the assassinations of Ms. Hill’s grandfather and her great-uncle, John F. Kennedy.