SANTIAGO, Chile — A judge convicted six men on Wednesday in the 1982 murder of former President Eduardo Frei Montalva of Chile, then the leader of the moderate opposition against the dictator Augusto Pinochet.
In an 811-page ruling, Judge Alejandro Madrid found that the men — a former security agent, four doctors and Mr. Frei’s driver — conspired to slowly poison Mr. Frei after he had surgery in a private clinic in Santiago, the capital, and then worked to conceal the autopsy report.
Mr. Frei, of the centrist Christian Democratic Party, served as president of Chile from 1964 to 1970. His government began a land reform program and took majority control of the copper industry, then in the hands of foreign corporations.
After initially supporting the 1973 military coup against his successor, Salvador Allende, Mr. Frei and his party soon became vocal opponents of the military junta because of widespread human rights violations. At the time of his death on Jan. 22, 1982, Mr. Frei was leading efforts to unite the moderate political opposition to oust General Pinochet.
“The regime decided that he had to be eliminated because he was an extremely dangerous figure for them, and put in motion a premeditated plan,” said his daughter Carmen Frei, the vice president of the Christian Democratic Party and the author of the 2017 book “Magnicide,” on the life and murder of her father.
Mr. Frei’s death, after a long and complicated post-surgical treatment, was deemed to be the result of natural causes at the time. However, in 2000, the Frei family received the first tips that pointed to the involvement of third parties, and Ms. Frei, then a senator, pushed for a formal investigation.
The case was opened in 2002 and Mr. Frei’s remains were exhumed two years later and sent to foreign labs, where they tested positive for toxic substances. They were exhumed again in 2016 for further analysis.
Judge Madrid, who had indicted the men in 2009, sentenced the former army doctor Patricio Silva to 10 years in prison. The former security agent Raúl Lillo and Mr. Frei’s longtime driver, Luis Becerra, were sentenced to seven years each. Three other defendants were sentenced to terms ranging from three to five years.
In a statement, President Sebastián Piñera conveyed his condolences to Mr. Frei’s family and expressed his “most indignant condemnation of such a cruel and vile act.”
Mr. Frei’s murder came at a time when opposition parties and the labor movement were regrouping after almost a decade of severe political repression. A month later, a prominent labor leader, Tucapel Jiménez, who was mobilizing a union movement against General Pinochet, was killed by security forces.
“The battle doesn’t end here,” said Mr. Frei’s son, also named Eduardo Frei, who was a president himself from 1994 to 2000. “Our next task is to establish the political responsibilities of high government officials at the time. This wasn’t the work of a few agents or doctors.”