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SANTIAGO (Reuters) - A Chilean judge convicted six people on Wednesday of assassinating former President Eduardo Frei Montalva during the 1973 to 1990 dictatorship of Augusto Pinochet.

Judge Alejandro Madrid sentenced the ex-president’s doctors, his chauffeur, an army officer and a former intelligence agent to between three and 10 years in jail for the poisoning of Frei in a Santiago clinic in 1982.

The judge determined that Frei had been drugged by Pinochet’s agents while the then 71-year-old recovered from hernia surgery in the clinic, according to court documents. Some of the doctors convicted then worked to cover up the poisoning, complicating the subsequent investigation.

Frei had initially supported Pinochet and the coup that overthrew socialist Salvador Allende in 1973. But he later soured on the military dictatorship and became one of the leaders of Chile’s pro-democracy movement, a move that apparently precipitated his murder.

Madrid originally brought charges against the six men now convicted of the crime in 2009, but they were later dismissed. In 2016, Madrid ordered the exhumation of Frei’s body and asked forensic experts to re-examine the remains.

The results confirmed that the former president was poisoned and strengthened the case against the accused.

Pinochet died in 2006 without ever facing a full trial for human rights abuses committed under his rule, when more than 3,000 people were killed or disappeared and around 28,000 people were tortured.

In recent years, prosecutions against dictatorship-era crimes have picked up pace, as support for Pinochet - once strong among conservative sectors of society - has faded.