Gen. Efraín Ríos Montt, who as dictator of Guatemala in the 1980s ordered fierce tactics to suppress a guerrilla insurgency and was later convicted of genocide and crimes against humanity, died on Sunday in Guatemala City. He was 91.

The cause was a heart attack, according to his lawyers, Jaime Hernández and Luis Rosales. The general had dementia and had suffered lung and heart problems in recent years.

In the panoply of commanders who turned much of Central America into a killing field in the 1980s, General Ríos Montt was one of the most murderous. He was convicted in 2013 of trying to exterminate the Ixil ethnic group, a Mayan Indian community whose villages were wiped out by his forces.

A Guatemalan judge found that the general had known about the systematic massacres, in the hillside hamlets of the El Quiché department, and had done nothing to stop them or the aerial bombardment of refugees who had fled to the mountains.