Mexico City — A federal tribunal has exonerated former President Luis Echeverría Álvarez of genocide charges stemming from a notorious massacre of student activists in 1968, closing the books on one of the most high-profile cases in Mexico’s recent history.

Mr. Echeverría, who is 87, had been under house arrest since 2006.

The court, which issued its ruling late Thursday, rejected federal prosecutors’ argument that an army crackdown on unarmed student protesters fit the legal definition of genocide. It also upheld previous rulings that the 30-year statute of limitations for genocide had expired.

Reports from eyewitnesses put the number of students killed in the Tlatelolco plaza, in downtown Mexico City, at between several dozen and 300. Federal prosecutors argued that Mr. Echeverría, who was interior minister at the time, had ordered the massacre, on October 2, 1968, to stamp out student unrest just days before the country played host to the Summer Olympics.

Mr. Echeverría, who was president from 1970 to 1976, has denied responsibility for the killings.

His lawyer, Juan Velásquez, broke the news of the court ruling to the former president late Thursday at his heavily guarded house in southern Mexico City. “Maybe there are people who say that history has already judged him,” Mr. Velásquez was quoted as saying in El Universal, a newspaper. “But no, it’s the courts that judge, and if the courts have absolved him, then that is how the case should go down in history.”

Mr. Echeverría was cleared in 2005 of genocide charges in a later crackdown on student protesters, in 1971, in which at least 11 people died. He is the first president in modern Mexican history to have been indicted in a criminal court.

Thursday’s ruling exhausted prosecutors’ legal possibilities in Mexico. It was not immediately clear whether they planned to appeal the decision to the Inter-American Court of Human Rights, in Costa Rica. —Marion Lloyd