WASHINGTON (AP) ― Six years after she was deemed cancer-free, Kara Kennedy accepted the Presidential Medal of Freedom on behalf of her father, just weeks before Sen. Edward Kennedy died battling a brain tumor.

However, her own lung cancer treatment ― surgery and grueling chemotherapy and radiation ― left her physically weakened, her brother Patrick Kennedy said. She died Friday at age 51 after her daily workout at a Washington health club.

“Her heart gave out,” said Patrick Kennedy, a former congressman from Rhode Island.”

“She’s with dad.”

In a telephone interview from her home in Boston on Saturday Joan Bennett Kennedy said she and her daughter were “best friends” who liked to take long swims together and walks on the beach. She said her daughter had fully recovered from her cancer and didn’t have any lingering health issues.

“She was very healthy. That’s why this is such a shock,” Joan Kennedy said.

Kennedy was a member of the Sport & Health fitness center, though spokeswoman Nancy Terry declined to release further details about the incident, citing member privacy.

Her ex-husband, Michael Allen, said Kara Kennedy frequently visited the club and went swimming every day if she could. He said details about her death would be released by the Edward M. Kennedy Institute for the United States Senate. He said funeral arrangements are being made.

“Insofar as I’m concerned her legacy is one of courage and grit and determination in the face of her own illness and in the face of many family tragedies and limitless, absolutely limitless, devotion to our children,” he said.

Kara Kennedy was born in 1960 to Edward and Joan Kennedy, just as her father was on the campaign trail for his brother John F. Kennedy during the presidential primaries.

The late senator wrote of his oldest child in his 2009 memoir, “True Compass,” that “I had never seen a more beautiful baby, nor been happier in my life.”

Later, she appeared with her father during his unsuccessful 1980 campaign for the Democratic presidential nomination, and she and her brother Edward Kennedy Jr. helped run the senator’s 1988 re-election campaign.