A Chilean court has sentenced three retired soldiers to 10 years in prison for their part in a horrific attack on two democracy activists who were doused with petrol and set on fire.

The 1986 attack on Rodrigo Rojas and Carmen Gloria Quintana was one of the most notorious torture cases in the 17-year military dictatorship of Augusto Pinochet.

On Thursday, Julio Castaner, Iván Figueroa and Nelson Medina were found guilty of murder and attempted murder. Eight other former soldiers received three-year prison sentences for acting as accomplices in the attack.

Rojas, a US resident, had returned to Chile to photograph popular protests against Pinochet. He and Quintana were captured by soldiers on 2 July 1986 during a two-day national strike.

Quintana and other witnesses described how she and Rojas were beaten, sprayed with petrol and set alight. They were then dumped on the outskirts of the capital Santiago where they were found by nearby residents and rushed to a hospital.

Rojas died from his burns, and Quintana was left disfigured, but attempts to investigate the case were stymied by a decades-long “pact of silence” among the former servicemen.

Pinochet himself suggested that the two activists had accidentally set themselves on fire while they were making petrol bombs.

The case was reopened in 2013, and, in 2014, a former army conscript identified the men who had participated in the attack

Quintana underwent lengthy treatment for severe burns at a Canadian hospital, and eventually became a Chilean diplomat after democracy was restored.

During the dictatorship of Pinochet between 1973 and 1990, some 3,000 people were killed or disappeared in Chile. Another 28,000 were victims of torture, including former president Michelle Bachelet.