Appeals judges in Chile said Monday ex-president Eduardo Frei's death four decades ago was not murder, overruling a 15-year investigation which concluded he was poisoned in hospital by agents of dictator Augusto Pinochet.
The panel of three judges cleared all six people convicted and sentenced to jail two years ago over Frei's death.
The ex-president's family and political party cried foul, however, and vowed to have the latest ruling overturned.
"We will point to the evidence that (investigating magistrate Alejandro) Madrid accumulated in the long investigation to show that it was murder," Frei's family said in a statement issued by their lawyers.
The family said they would appeal the decision to Chile's highest court.
Frei, president from 1964 to 1970, died aged 71 of a sudden infection in January 1982 while hospitalized for hernia treatment during Pinochet's brutal rule.
Pinochet took power 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.
Two years ago, Madrid -- who under Chile's system handled both the investigation and the judgment -- found six people guilty of Frei's "homicide by poisoning after a surgical intervention."
The 15-year investigation saw Frei's body exhumed twice for forensic examination, the last time in 2016 to determine what type of toxin was used after Madrid determined poisoning to have been the cause of death.
- Poison allegations -
Madrid ruled in January 2019 that medical doctor Patricio Silva Garin had administered the poison that caused Frei's death, and sentenced him to 10 years in jail.
Another doctor, Pedro Valdivia, was sentenced to five years' imprisonment as an accomplice.
Luis Becerra Arancibia, the ex-president's driver, and intelligence agent Raul Lillo Gutierrez were each handed a seven-year sentence.
Two physicians specializing in palliative care, Helmar Rosenberg and Sergio Gonzalez, got three years each for covering up the crime.
Garin, Rosenberg and Gonzales have since died. The sentences were suspended pending the outcome of the appeal.
On Monday, the appeals court in Santiago said the facts of the case were inconsistent with murder.
There was no evidence, the judges said, that Frei's death was the result of "the willful or culpable action" of anyone, and his passing could be "reasonably explained" by ill health that required surgery.
Fuad Chahin, president of Frei's Christian Democratic Party said Monday's ruling was "wrong."
And party senator Ximena Rincon said on Twitter: "We know what happened... Frei Montalva was murdered and we hope that the Supreme Court will establish this."
Chilean Nobel laureate and Pinochet critic Pablo Neruda died in the same clinic as Frei 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.
More than 3,200 people were killed under the Pinochet regime, and some 38,000 people were tortured.
More than 1,000 former opponents remain missing to this day.