Frank Sinatra Jr. died of a massive heart attack Wednesday afternoon in Florida, TMZ reported.

The 72-year-old musician was in Daytona Beach for a concert when he was taken to a hospital, feeling lightheaded. He seemed to be recovering there when he died suddenly, the entertainment site wrote, citing a Sinatra family representative.

“The Sinatra family mourn the untimely passing of their son, brother, father, uncle, Frank Sinatra, Jr. … Sleep warm, Frankie,” tweeted his sister, Nancy Sinatra, who also had a successful music career in the 1960s. He had one son, Michael, age 29.

While not as popular as his father (few people were), Mr. Sinatra recorded a half-dozen albums and appeared on several TV shows from the Smothers Brother and Dean Martin shows to “Adam-12.” He also worked as his father’s conductor and musical director in the twilight of the legendary singer’s career.

The most famous incident in his life may have been his Lake Tahoe kidnapping in 1963, when he was found walking the streets of Bel Air, Calif., after a $240,000 ransom had been paid.

Mr. Sinatra acknowledged that being the son of one of the most popular entertainers of the 20th century was both a blessing and a curse — opening doors while setting an impossible and obvious standard of comparison.

“You have to work three times harder than the guy off the street,” he told Nat Hentoff for the book “The Other Frank Sinatra.”

“Sinatra, Jr. might have had an easier time establishing himself had he gone into real estate. But his show made me awfully glad he decided music was his calling. There aren’t too many singers around with Sinatra’s depth of experience in big band music, or his knowledge of the classic American songbook. There are even fewer with such real feeling for the lyrics of a song, and such a knack for investing a song with style and personality,” wrote music critic Richard Ginell in a 2003 Daily Variety review of his concert.