WASHINGTON (AP) -- Kara Kennedy accepted the Presidential Medal of Freedom with tears welling in her eyes on behalf of her ailing father at a 2009 White House ceremony but also managed a smile as Sen. Ted Kennedy's life was honored. After the Democratic senator died two weeks later following a battle with brain cancer, his only daughter read a psalm at his funeral Mass in Boston, moving many.
The eldest of the senator's three children, Kara Kennedy died Friday at age 51, collapsing after her daily workout at a Washington health club. The cause of death wasn't immediately released.
Though she had never sought elected office like many in her famously political family, she helped with her father's presidential and Senate campaigns and heeded his call to give back through public service. She worked as a filmmaker and in television and was active in an array of causes from the arts to battling fetal alcohol syndrome.
She herself had been diagnosed with lung cancer in 2002, but underwent surgery the following year that doctors had said was successful.
But her brother, Patrick Kennedy, said her cancer treatment -- surgery and grueling chemotherapy and radiation -- left her physically weakened.
"Her heart gave out," said Patrick Kennedy, a former Democratic congressman from Rhode Island.
"She's with dad."
In a telephone interview from her Boston home Saturday, Joan Bennett Kennedy said she and her daughter were "best friends" who liked to take long swims together and enjoyed walks on the beach. She said her daughter had fully recovered from cancer and didn't have any lingering health issues.
"She was very healthy. That's why this is such a shock," Joan Kennedy said.
Kara Kennedy was a member of the Sport & Health fitness center, though spokeswoman Nancy Terry declined to release further details about the death, citing member privacy. A spokeswoman for the District of Columbia medical examiner said in an email that the cause of death was pending.
Kennedy's ex-husband, Michael Allen, said she frequently visited the club and went swimming every day if she could. He said funeral arrangements were being organized.
"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.
A graduate of Tufts University, Kara Kennedy also helped produce several videos for Very Special Arts, an organization founded by her aunt Jean Kennedy Smith. She also served as a board member for the Edward M. Kennedy Institute; director emerita and national trustee of the John F. Kennedy Library Foundation; and as a national advisory board member for the National Organization on Fetal Alcohol Syndrome.
Her lung cancer diagnosis came in 2002, and the prognosis was grim. But the family refused to accept that, the senator wrote. She had an operation in 2003, and Edward Kennedy accompanied his daughter to chemotherapy treatments.
"Kara responded to my exhortations to have faith in herself," he wrote later. "Today, nearly seven years later as I write this, Kara is a healthy, vibrant, active mother of two who is flourishing."
Her children, Grace and Max, are now teenagers.
Kara Kennedy's two brothers have dealt with health issues of their own: Edward Kennedy Jr. lost a leg to bone cancer as a child, and Patrick Kennedy had surgery in 1988 to remove a non-cancerous tumor that was pressing against his spine.
"Her magnificent strength in her successful battle with lung cancer was a quiet inspiration to all of us and provided her family and fellow patients with hope," the Edward M. Kennedy Institute said in a statement.
In August 2009 Kara Kennedy accepted the Presidential Medal of Freedom, the nation's highest civilian honor, on behalf of her father. Tears filled her eyes as the senator's accomplishments were read aloud. She smiled when President Barack Obama, whom Ted Kennedy had endorsed in 2008, put his arm around her shoulder in a comforting way.