Saoirse Kennedy Hill, 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.

The 22-year-old daughter of Courtney Kennedy Hill, died at the family’s compound in Hyannis Port, Massachusetts, on 1 August. Emergency responders were called to the compound in the afternoon, and Hill was taken to Cape Cod Hospital in Hyannis, where she was pronounced dead at 3:14pm, according to the death certificate.

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

The death certificate said that Hill’s death resulted from mixing alcohol with methadone, a medication used to treat opioid use disorder; diazepam and nordiazepam, which have sedative effects; and fluoxetine and norfluoxetine, which are used to treat depression.

The manner of death was ruled an accident. A member of the staff in the town clerk’s office in Barnstable, Massachusetts, read the certificate over the phone.

A Day That Shook The World: John F. Kennedy assassinated Show all 8 1 /8 A Day That Shook The World: John F. Kennedy assassinated A Day That Shook The World: John F. Kennedy assassinated President John F. Kennedy (1917 - 1963) and his wife Jacqueline Kennedy ride with secret agents in an open car motorcade shortly before the president was assassinated in Dallas, Texas, November 22, 1963. Getty Images A Day That Shook The World: John F. Kennedy assassinated Cliff Michelmore pointing to the path of gun shots which killed President John F Kennedy, on a model reconstruction of the Dealey Plaza, Dallas, in the USA. Getty Images A Day That Shook The World: John F. Kennedy assassinated A man holding the rifle used by Lee Harvey Oswald in his assassination of the American President John F Kennedy. Getty Images A Day That Shook The World: John F. Kennedy assassinated circa 1965: The Texas School Book Depository at Dallas, Texas, from which US president John F Kennedy is thought to have been shot. Getty Images A Day That Shook The World: John F. Kennedy assassinated 23rd November 1963: A woman buys a newspaper from a street vendor for the latest headlines concerning the assassination of John F Kennedy. Getty Images A Day That Shook The World: John F. Kennedy assassinated The family and friends of the assassinated American President John F Kennedy (1917 - 1963) mourning his death. Getty Images A Day That Shook The World: John F. Kennedy assassinated Members of the Kennedy family at the funeral of assassinated president John F. Kennedy at Washington DC. From left: Senator Edward Kennedy, Caroline Kennedy, (aged 6), Jackie Kennedy (1929 - 1994), Attorney General Robert Kennedy and John Kennedy (1960 - 1999) (aged 3). Getty Images A Day That Shook The World: John F. Kennedy assassinated 22nd November 1963: US statesman John F Kennedy, 35th president of the US, and his wife Jackie Kennedy travelling in the presidential motorcade at Dallas, before his assassination. Getty Images

Hill’s father, Paul Michael Hill, was one of the Guildford Four, who were falsely accused of involvement in IRA bombings. He was imprisoned for 15 years before his conviction was overturned.

Saoirse Kennedy Hill had been enrolled at Boston College, where she was a communication major and vice president of the College Democrats. In high school, at Deerfield Academy, she had written an essay for the school newspaper about struggling with depression, describing “bouts of deep sadness that felt like a heavy boulder on my chest.”