A Mexican court has issued an arrest warrant for former President Luis Echeverria, accused of genocide for an alleged massacre of students in 1968.
Echeverria may be the first ex-president to face criminal charges
The decision came two days before Mexicans vote to elect a new president.
Mr Echeverria, 84, allegedly ordered the killing of student protesters in 1968, days before the Olympic Games opened in Mexico City.
Prosecutors say up to 300 people may have died when government agents hidden among regular soldiers opened fire.
Mr Echeverria was interior minister in 1968 at the time of the killings in Tlatelolco Square.
Mr Echeverria, who is frail and in poor health, is expected to be held under house arrest
He has acknowledged that some died in Tlatelolco Square in 1968.
But he denies allegations that he posted snipers on scores of buildings to fire into the crowd and gave orders to fire.
His lawyer, Juan Velasquez, told the Associated Press the killings were not part of an extermination policy, adding that Mr Echeverria would not serve time in prison because of his age.
Mexican students still mark the 1968 Tlatelolco massacre
Special prosecutor Ignacio Carillo has tried and failed twice to bring genocide charges against Mr Echeverria.
Hundreds died or disappeared during Mexico's "Dirty War" on leftists under Mr Echeverria's presidency between 1970 and 1976.
Outgoing Mexican President Vicente Fox, who leaves office in December, has vowed to bring to justice those responsible for killings and oppression in the country's past.
A recent leak of a draft version of a report on the Dirty War was rejected as invalid by Mr Fox, who has said he will only comment on a final version, as yet unreleased.