John F. Kennedy's nephew Christopher Lawford dead at 63

The Kennedy family is mourning the loss of Christopher Lawford.

Lawford's rep confirmed the sad news of his death to ET on Wednesday. Aside from being known for his famous family -- Lawford was the son of the late John F. Kennedy's sister, Patricia Kennedy, and actor Peter Lawford -- Lawford had acting roles on multiple hit TV shows including Frasier, General Hospital and All My Children. His movie roles included Terminator 3: Rise of the Machines and Thirteen Days, which interestingly enough, featured his famous uncles, John and Robert Kennedy, as characters.

Lawford was 63 years old.

According to Lawford's cousin, Patrick J. Kennedy, the actor died of a heart attack.

"The recovery community lost a founding father today," he wrote, referring to Lawford's openness about battling drug addiction. "My cousin, Chris Lawford, passed away from a heart attack yesterday evening. To the world he was an author, actor, & activist, but to the recovery community he was a pioneer -- living proof that long-term recovery was possible."

Kerry Kennedy, another one of Lawford's cousins, also tweeted about the family's loss on Wednesday.

"We mourn the loss of my cousin Christopher Lawford, Rest in Peace," Kennedy wrote. "Pictured here with our family at the 2004 Democratic Convention and with his wonderful son, David."

According to TMZ, Lawford had a medical emergency at a yoga studio on Tuesday night and later died. The outlet quotes sources as saying his death appears to be from "natural causes."

Aside from being an actor, Lawford released a New York Times bestselling memoir, Symptoms of Withdrawal: A Memoir of Snapshots and Redemption, in 2005, which detailed both his life as a Kennedy and his battle with drug addiction in the '70s and '80s. In 2009, he released Voices From the Front Lines of Addiction and Recovery, which featured essays from Tom Arnold, Alec Baldwin, Jamie Lee Curtis, Richard Dreyfuss, Anthony Hopkins and more.

In 2015, Lawford candidly talked to Parade magazine about battling addiction.

"I was the product of an addictive perfect storm," he said. "We had an entirely different culture in 1969 when I first used drugs. Experimentation, permissiveness and, frankly, ignorance of what this disease is. Another piece of the storm was severe trauma in adolescence. The fact that any of us in [the Kennedy] family survived what we went through is a miracle. [The trauma] obliterated any safety I thought I had and my ability to have relationships with anybody. That’s what made me an addict."

"Addiction is a brain illness, and it’s not limited to one demographic or another," he continued. "You can be an alcoholic in the White House or in the poor house. [Addiction] runs in both sides of my family."

Lawford is survived by his wife, Mercedes Miller, and his three children from his first marriage to Jeannie Olsson.