Mexico's Supreme Court has ruled that a former president accused of human rights abuses cannot be tried for an attack on students in 1971.
The court approved a 30-year time limit in the case, exempting Luis Echeverria from prosecution on genocide charges.
The decision is a setback for current Mexican leader Vicente Fox, who has pledged to punish human rights crimes.
Hundreds died between 1970 and 1976 as Mr Echeverria's government ran a "dirty war" against leftists.
The court voted 4-1 in favour of applying the time limit on the crime, upholding a decision by a lower court.
Mr Echeverria, now 83, is the first former Mexican president to face the possibility of charges for human rights abuses allegedly committed during his time in power.
Students killed
Special prosecutor Ignacio Carrillo could decide to pursue further charges against Mr Echeverria.
The Supreme Court confirmed it would consider a series of other arguments put forward by Mr Carrillo.
According to Mr Carrillo, the former president ordered men loyal to the government to attack protesters during a student demonstration in June 1971.
Defence lawyers admit that 11 people died in the attack, although Mr Carrillo insists that up to 50 died in the "Corpus Christi massacre".
Mr Echeverria was accused of using a gang of armed government thugs, known as the "Halcones", or Falcons, to systematically attack enemies of the government during his time in office.
Ten other people, including former government members and five members of the Halcones, have been charged with involvement in the alleged attack.
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