Prosecutors sought charges against a former Mexican president in the deaths of student demonstrators more than 20 years ago, a first for a country whose leaders have long enjoyed impunity.

The case against Luis Echeverria (search) threatens to create a crisis in President Vicente Fox's already troubled relationship with Congress. Echeverria's Institutional Revolutionary Party (search) holds the largest bloc of seats and could stop cooperating with Fox if the charges go through.

Special Prosecutor Ignacio Carrillo said he presented findings in a two-year investigation to a federal judge, asking the court to confirm charges of genocide, but he did not say against whom. He said the law prohibited him from saying more.

Echeverria's attorney, Juan Velazquez, confirmed the genocide charges had been sought against the former president, and two of his other clients, former Interior Secretary Mario Moya and former Attorney General Julio Sanchez Vargas. Local media reported at least seven other officials could also be charged.

Under Mexican law, prosecutors propose charges and arrest warrants to a court, which must confirm both.

The judge's decision will be closely watched in a country struggling to investigate and punish government repression against student, leftist and guerrilla groups in the 1960s and 1970s. Never before in modern Mexican history has a former president faced criminal charges for his actions in office.

"Presidents in the history of Mexico have been sort of untouchable, rarely held accountable," said Eric Olson, of Amnesty International. "By pushing forward in this hopefully this will lead to more public acceptance of the justice system and lend more credibility to the system."

Fox made uncovering past abuses a priority when he took office four years ago. His effort has angered powerful figures in Echeverria's party, the PRI, which ruled Mexico from 1929 until Fox's election in 2000.

Carrillo said "dozens" of students were killed when a civilian-clothed government force called the "Halcones," or Falcons, attacked student demonstrators in Mexico City (search) on June 10, 1971.

He said that fell under the definition of genocide in a 1967 Mexican law because the victims were "a national group of political dissidents" who were "partially destroyed through the illegal use of physical force."

Echeverria's attorney, Juan Velazquez, has said several times it was a "hallucination" to describe the case as genocide. "There was no genocide ... in the sense of a state policy of exterminating a population," he told The Associated Press. He put the death toll at 11.

Reporters gathered outside Echeverria's house Friday, but he did not appear. Velazquez said Echeverria was waiting to hear from the judge. In the past the former president has denied planning or knowing about the clash before it occurred.

Carrillo presented evidence to a court Thursday evening. The judge has 24 hours to respond, but it was unclear when he formally received notification and whether his decision would be made public.

At a news conference, Carrillo called for the nation to move forward, "illuminating the recent past with the law, without bitterness, resentment or useless rancor."

"We all have the right to know," said Carrillo, who also is in charge of prosecuting crimes related to a larger 1968 massacre of students and to the general "dirty war" between government forces and radical guerrillas.

Even if a judge orders the arrest, Echeverria would likely face only house arrest under Mexican law because of his age.

Political analyst and human rights activist Sergio Aguayo praised Carrillo's action, saying it puts in danger "the whole system of repression that was put in place by the PRI and in which many of (the PRI members) could be implicated."

The party, which has the largest bloc in Congress, recently threatened to restrict cooperation with Fox if Echeverria is charged. Party leaders said Friday they had named a commission to study the charges.

"We will not accept any attempt by the state to annihilate political adversaries or to connect these events to electoral processes," the party said in a statement. Fox has said he will not interfere with the legal investigation.

Velazquez argued that a 30-year statute of limitations on a crime of genocide would have expired on June 10, 2001. Carrillo said Friday that for technical reasons, the 30-year countdown had been interrupted and charges were still timely.

The two attorneys also disagreed over whether genocide has a statute of limitations.