The actress, who died in April, was exhumed for tests to investigate claims she was poisoned.
But in a statement broadcast by Televisa news in Mexico, Benjamin Felix, the star's younger brother, said the initial results resolved remaining doubts about the cause of death.
"There was no pressure," he said. "We were after the truth and we found it."
The body was exhumed on Thursday under police guard, as photographers outside the cemetery jostled for pictures.
Benjamin Felix previously said he believed she had been poisoned, despite a report from her physician which said she died of a heart attack on 8 April, her 88th birthday.
Mexico City prosecutor Bernardo Batiz told the TV the forensic examination would continue for about one week.
The death of Felix, who charmed cinema audiences throughout the Hispanic world for decades, brough Mexico to a virtual standstill.
Correspondents were unable to say someone might have tried to kill her.
But brother Benjamin Felix says he and other family members were prevented from seeing her body after she died.
Strong and silent
Felix was renowned as a femme fatale and the one-time lover of the painter Diego Rivera. Her four turbulent marriages also regularly made headlines.
She made 47 films in her career though few of them were shown widely outside Spanish-speaking nations.
Felix often portrayed strong, silent women, endowed with intelligence and a voluptuous glamour.
Born in 1914 in Alamos, northern Sonora state, her real name was Maria de los Angeles Felix Guerena but she became affectionately known as La Dona, or The Dame in Mexico after her film hit Dona Barbara in 1943.
She also had a home in Paris where she spent half the year and in 1996 France awarded her the nation's highest distinction, the National Order of Arts and Letters.
Felix last starred in La Generala (The Lady General) in 1970.