Saoirse Kennedy Hill, daughter of the Guildford Four's Paul Hill and granddaughter of Robert F Kennedy, died from an accidental drug overdose, according to her official death certificate.
The 22-year-old was found unresponsive at the Kennedy family's Cape Cod residence in August.
She was pronounced dead at Cape Cod Hospital.
According to the Boston Herald, she had a mix of methadone, fluoxetine, norfluoxetine, diazepam, nordiazepam, and alcohol in her system at the time of her death.
Fluoxetine is an antidepressant, and diazepam is used to treat anxiety and alcohol withdrawal symptoms.
The compound where she was found has around six homes on Nantucket Sound in Hyannisport and famously served as President John F. Kennedy's summer White House.
The Boston College student was expected to graduate in 2020.
She was the only daughter of Mr Hill and Robert and Ethel Kennedy's fifth child, Courtney.
Ms Kennedy Hill had written publicly about her struggles with mental health and a suicide attempt in a 2016 opinion piece for the Deerfield Scroll, the student newspaper for the Deerfield Academy boarding school in Massachusetts.
