Kara Kennedy, the daughter of the late U.S. Senator Edward Kennedy, died Friday of heart failure. She was 51.

The Edward M. Kennedy Institute for the United States Senate in Boston told the New York Times she died of a heart attack. It happened after a workout at a health club in the Washington area, Reuters reports.

Kennedy Allen, who was Senator Kennedy's only daughter, was reportedly in remission after she had a malignant tumor removed from her lung in 2003, her brother, Patrick Kennedy, a former Rhode Island congressman, told BBC. He said the past health problem had physically weakened her.

The cancer treatment "took quite a toll on her and weakened her physically," he told the AP. "Her heart gave out," he added. "She's with dad."

Their father, the late senator from Massachusetts, died in August 2009 after battling brain cancer. He was 77.

Kara Kennedy was a television producer and filmmaker. She was married to Michael Allen and the mother of two teenage children, Max and Grace.