A copy of a military file dating from the 1980s, complete with maps, telegrams and hand-written patrol reports about an operation known as "Plan Sofia," was mailed anonymously to President Alvaro Colom last year.
Colom's government verified its authenticity and passed it to the attorney general's office, which has a long-running case against the Central American country's former dictator, Efrain Rios Montt, accusing him of ordering the murder of thousands of civilians.
The document was also turned over for use in a parallel case in Spain, brought by Mayan human rights activist Rigoberta Menchu, which accuses Rios Montt of carrying out genocide during his 1982-83 rule.
Nearly a quarter of a million people, mostly native Mayans, died during the 36-year-long civil war pitting leftist guerrillas against security forces.
Rios Montt, now 83, still serves in Guatemala's Congress.
A U.N.-backed Truth Commission report found the army committed 85 percent of the killings, but this is the first time a military document might be able to link the highest chain of command to human rights violations in a court of law.
"These documents paint a picture of command responsibility," said Andrew Hudson of the Washington-based group Human Rights First. "When put together they show Rios Montt and the top commanders were aware of and were directing a policy which the United Nations says constituted genocide," he told Reuters.
Colom pledged to address war-time abuses after taking office in 2008. His uncle, Manuel Colom Argueta, was a prominent leftist politician killed by a military ambush in 1979 at the height of Guatemala's 1960-1996 civil war.
He promised to open sealed military archives that date back to 1954, when a U.S.-backed coup toppled Guatemala's democratically elected president, despite the army's claims that opening the files would threaten national security.
The document reveals details about the military's scorched earth campaign aimed at wiping out guerrilla sympathizers.
"Plan Sofia" was based on a counter-insurgency strategy to attack towns providing food and shelter to guerrilla fighters as a way to "drain the water from the fish," Colom's government said in a statement after making the document public.
Women, children and old people were routinely beaten, raped, tortured and killed by soldiers during these raids, according the country's truth commission.
