A Mexican judge has refused to issue an arrest warrant for a former president accused of genocide.
Luis Echeverria has never admitted any personal responsibility
The judge said the charges against Luis Echeverria did not amount to genocide.
Mr Echeverria, now 83, was accused of ordering a massacre of student protesters in 1968, days before the Olympic Games opened in Mexico City.
As many as 300 people may have died when government agents hidden among regular soldiers opened fire on students, prosecutors allege.
Judge Ranulfo Castillo said several other officials charged with Mr Echeverria over the massacre could not be prosecuted because the statute of limitations - a legal time limit for bringing a case to trial - had expired.
In Mr Echeverria's case, Judge Castillo said the 1968 killings could not be classed as genocide, which is defined as an attempt to destroy an entire ethnic or national group.
"I won," defence lawyer Juan Velasquez told the Reuters news agency. "It's the end of the process."
A spokesman for Special Prosecutor Ignacio Carillo told the agency they would appeal against the ruling.
Mr Echeverria also faced a kidnapping charge over the 1969 disappearance of activist Hector Jaramillo, but this was thrown out by the judge because of a lack of evidence.
Mr Echeverria is the first former Mexican president to have faced the possibility of charges for human rights abuses allegedly committed during his time in office.
Mexican students still mark the 1968 Tlatelolco massacre
He was serving as interior minister and head of national security at the time of the massacre.
Mr Carrillo filed the charges after a judge ruled in July that Mr Echeverria could not be charged over separate killings in 1971.
Mr Carrillo said his investigations had exonerated the Mexican army of responsibility for the massacre, and said he hoped to end 37 years of "impunity and injustice".
Mr Echeverria has acknowledged that some died in Tlatelolco Square in 1968.
He denies prosecution allegations that he posted snipers on scores of buildings to fire into the crowd and gave orders to fire.
Mexico's current President, Vicente Fox, has vowed to punish public figures found to be responsible for killings and oppression in the past.
Hundreds died or disappeared during Mexico's "Dirty War" on leftists under Mr Echeverria's leadership between 1970 and 1976.