MEXICO CITY — A Mexican judge refused to issue an arrest warrant Saturday against former President Luis Echeverria on charges of genocide in the 1971 killings of student protesters.

Prosecutors said they will appeal the decision in the case against Echeverria, the first former leader to face criminal charges in Mexico's modern history.

"We will exhaust all pertinent legal recourse," said Special Prosecutor Ignacio Carrillo, adding that he plans to ask the Supreme Court to hear the case.

Federal Judge Jose Cesar Flores did not explain his decision, and Carrillo did not reveal which of two defense arguments the judge upheld: whether the statute of limitations on the case had run out, or whether there was insufficient evidence to link Echeverria to the killings.

Flores had one day to review the case file, and Carrillo said the judge "did not fully analyze the evidence contained in the 14 volumes, consisting of 9,382 pages, probably because of time constraints."

The charges against Echeverria have threatened to create a crisis in President Vicente Fox's already troubled relationship with Congress. Echeverria's Institutional Revolutionary Party holds the largest bloc of seats and had threatened to stop cooperating with Fox if the arrest warrant was issued.

International observers said the decision could weaken the little faith Mexicans have in their justice system, generally perceived as corrupt and inept.

"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."

Echeverria's attorney, Juan Velazquez, said Carrillo also has sought the arrest of former Interior Secretary Mario Moya and former Attorney General Julio Sanchez Vargas.

The prosecutor said "dozens" of students were killed in the 1971 clash, when a civilian-clothed government force called the "Halcones," or Falcons, attacked student demonstrators in Mexico City. He said the crime fit the description of genocide under a 1967 Mexican law.

Velazquez has said only 11 people were killed in the clash, and that charges of genocide are not applicable because of a 30-year statute of limitations. He also argued there was not enough evidence.

The true number of dead in the case may never be known. Mexico's former national security adviser, Adolfo Aguilar Zinser, suggested that Carrillo had fixed on genocide charges in his zeal to find a way to prosecute the 82-year-old Echeverria, who governed Mexico from 1970-76.

Carrillo maintains the statute of limitations does not apply to genocide.

Echeverria, as interior secretary in the 1960s and as president in the '70s, allegedly fought a decade-long counterinsurgency campaign against the student pro-democracy movement and against violent leftist guerrilla groups.

That campaign has been compared to the "dirty wars" in Chile and Argentina in the 1970s, but it was a smaller phenomenon in Mexico, where the death toll probably ran in the hundreds — not thousands — killed.

The strategy of prosecuting political crimes on grounds of genocide was pioneered by Spanish Judge Baltasar Garzon, in his investigation of the abuses in Argentina and Chile. As a crime against humanity, genocide is not subject to usual procedural limits.

Echeverria is implicated not only in the 1971 clash but also in a larger number of killings at a 1968 demonstration leading up to that year's Olympics in Mexico City. It is also unclear how many were killed in that confrontation.

The former president has repeatedly denied planning or having any advance knowledge of the 1971 clash or any other confrontations.

Victims' activists hold out little hope high-ranking officials will be brought to justice.

"Charges come and go. Arrest warrants come and go," said Rosario Ibarra, who has led a 30-year campaign for justice after her son, an alleged guerrilla leader, disappeared in 1975. "They get injunctions, and in the end nothing is ever done."

But Ibarra, who has frequently confronted Echeverria personally about the crimes, vowed "we will not rest until we find out what happened to our loved ones."