The Mexican government and military committed "crimes against humanity" in the so-called "dirty war" against left-wing rebels, a leaked report says.
Hundreds of activists disappeared during the "dirty war"
The report was prepared for current President Vicente Fox but has not been released. A US NGO has printed material saying Mexicans had a right to know.
The army kidnapped, tortured and killed hundreds of rebel suspects, says the report, which covers 1964 to 1982.
Mexico's special prosecutor says the report is biased and has been revised.
The draft report's authors write: "The authoritarian attitude with which the Mexican state wished to control social dissent created a spiral of violence which... led it to commit crimes against humanity, including genocide."
They say they base their findings partly on declassified military, police and interior ministry documents and list for the first time the names of officers allegedly involved in the abuses.
The report says that units detained or summarily executed men and boys in villages suspected of links to rebel leader Lucio Cabanas.
Detainees were forced to drink gasoline and tortured with beatings and electric shocks, it says.
Bodies of dozens of leftists were dumped in the Pacific Ocean during helicopter "death flights" from military bases in Acapulco and elsewhere.
Mr Fox set up an office in 2002 to probe possible human rights violations under the administrations of Presidents Diaz Ordaz (1964-70), Echeverria (1970-76) and Lopez Portillo (1976-82).
The office presented the report to the special prosecutor investigating past abuses on 15 December but it was not released.
The Washington-based National Security Archive, a research institute on international affairs, has posted what it says is the draft report on its website.
Kate Doyle, director of the archive's Mexico Project, criticised the failure to make the report public.
"[This] is a state of affairs reminiscent of Mexico's past, when citizens were routinely shut out of civic participation by a government determined to keep them in the dark," she said.
The Mexican special prosecutor, Ignacio Carrillo Prieto, has said the draft report is biased and places too much blame on the military without pointing to the abuses committed by the rebels.
He says the president is to be given a revised version on Monday that will later be published.
Mr Carillo has tried unsuccessfully to bring genocide charges against Mr Echeverria for allegedly ordering a massacre of student protesters in 1968. The ex-president has denied any wrongdoing.