SANTIAGO, Chile (AP) - A Chilean judge ruled Monday that former President Eduardo Frei Montalva was assassinated nearly 30 years ago and his killing covered up by people linked to the dictatorship of Gen. Augusto Pinochet. Six people were charged in the case.

Frei, who preceded Salvador Allende as Chile’s president and later became a leading critic of the military dictatorship, died in 1982.

An autopsy report blamed septic shock after stomach hernia surgery, but a new autopsy this year by University of Chile pathologists identified two chemicals in his body that attack the digestive system - one that is used in mustard gas and another found in rat poison.

"He was injected with toxic substances, which produced other complications that deteriorated his immune system," Judge Alejandro Madrid told reporters Monday. "That was the cause of death."

At the time of his death, Frei, 71, was investigating human rights violations by the dictatorship of Pinochet, who toppled Allende in a 1973 coup.

"This goes to show that though it may take a while, justice in Chile always prevails," President Michelle Bachelet said of the judge’s ruling.

Madrid filed murder charges against one of Frei’s doctors who was involved in the surgery, Frei’s driver and a civilian army employee.