SANTIAGO, CHILE -- An appeals court in Chile ruled Monday that ex-president Eduardo Frei's death in 1982 was not murder, clearing six former agents of dictator Augusto Pinochet's regime who were convicted of "homicide by poisoning" and jailed.

In January 2019, a judge sentenced the six to between three and 10 years' imprisonment following a 15-year investigation that saw Frei's body exhumed twice for forensic examination.

Frei, president from 1964 to 1970, died of a sudden infection in January 1982 while hospitalized for hernia treatment. He was 71.

Pinochet became Chile's dictator soon after a military coup that killed Frei's successor, Salvador Allende, on September 11, 1973.

Frei died at a time that he and his Christian Democratic Party were gaining strength as the main opposition.

Judge Alejandro Madrid in 2019 found six people -- including four medical personnel -- guilty of Frei's "homicide by poisoning after a surgical intervention."

Madrid ruled that medical doctor Patricio Silva Garin had administered the poison that caused Frei's death, and sentenced him to 10 years in jail.

But an appeals court in Santiago Monday said the facts of the case were inconsistent with murder.

Chilean Nobel laureate and Pinochet critic Pablo Neruda died in the same clinic in 1973.

A team of international experts ruled in 2017 that Neruda did not die of the prostate cancer he was being treated for, but could not conclusively determine that he had been assassinated.