Washington DC - Ethel Kennedy, a tireless advocate for human rights and widow of assassinated politician Robert F. Kennedy, died Thursday at the age of 96, her family said.

"It is with our hearts full of love that we announce the passing of our amazing grandmother," former congressman Joe Kennedy III said on social media.

Ethel Kennedy, who died of complications from a stroke, was a matriarch of a family that counted former president John F. Kennedy – also assassinated – and Senator Ted Kennedy among its ranks.

She was also the mother of Robert F. Kennedy Jr., a vaccine conspiracy theorist whose failed third-party presidential bid and endorsement of Republican Donald Trump cast a shadow over the family's status in American politics and the Democratic Party.

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Born Ethel Skakel in Chicago in 1928, she met her future husband at the age of 17.

Robert Kennedy – known as Bobby or RFK – would go on to serve as US attorney general in his brother John's administration and later as a senator representing New York.

Five years after JFK's assassination, Robert Kennedy was himself shot dead while running for president in 1968. Six months after his death, Ethel Kennedy gave birth to the couple's 11th child.

She went on to found Robert F. Kennedy Human Rights, an advocacy organization notable for its work on freedom of expression around the world.

In 2014, then-president Barack Obama honored her with the Presidential Medal of Freedom, the highest civilian distinction in the US.

President Joe Biden led tributes that swiftly poured in for Kennedy Thursday, calling her "a hero in her own right."

"Kennedy was an American icon – a matriarch of optimism and moral courage, an emblem of resilience and service," Biden said in a statement, adding that she had "stood up for human rights around the world with her signature iron will and grace."

Former President Obama hailed her as an "extraordinary" woman, "a dear friend with a passion for justice [and] an irrepressible spirit."

Kennedy's niece, journalist Maria Shriver, posted on X that her aunt was "fearless, funny, smart, incredible athlete, patriot, madly in love with my Uncle Bobby, and devoted to making our country better in every way."

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Praising her "lifetime's work in social justice and human rights," Joe Kennedy III said, "we are comforted in knowing she is reunited with the love of her life, our father, Robert F. Kennedy."

She is survived by nine children, 34 grandchildren and 24 great-grandchildren, the former congressman said.