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 would 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 had sought the arrest of former Interior Secretary Mario Moya and former Atty. Gen. 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.