He was found on Thursday morning and is said to have suffered from depression. A nuclear physicist by training, Castro Diaz-Balart, 68, was the offspring of a family that combined the aristocratic class that ruled pre-revolutionary Cuba with the man who expelled them after taking power.
Mr Castro Diaz-Balart was born to Mr Castro's aristocratic first wife Mirta Diaz-Balart. Following much similarity in looks with his father he was often addressed as "Fidelito".
In a letter to his sister, Fidel Castro once wrote, "I refuse even to think that my son may sleep a single night under the same roof sheltering my most repulsive enemies and receive on his innocent cheeks the kisses of those miserable Judases".
Diaz-Balart studied nuclear physics and was executive secretary of Cuba's Nuclear Energy Commission from 1980 until his dismissal in 1992, The Miami Herald reported. He had five half brothers from Castro's second marriage, and a half sister from another one of his father's relationships.
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After the divorce of his parents, Fidelito spent some of his early years in the United States, before his father managed to bring him back to the island.
Cuba halted its plant plans that same year because of a lack of funding after the collapse of Cuba's trade and aid ties with the ex-Soviet bloc and he largely disappeared from public view appearing at the occasional scientific conference or diplomatic event.
He reportedly served as Scientific Advisor to the Council of State and Vice President of the Academy of Sciences of Cuba at the time of his death, according to Granma.
Popularly called "Fidelito", he was the first-born of the former President, who died in November 2016.