Just days earlier, human rights advocates had been praising Ignacio Carrillo Prieto, the government's dirty-war prosecutor, for bringing murder charges against Mr. Echeverría, the first president in Mexico's modern history to face criminal indictments. Mr. Carrillo's indictment alleges that Echeverría ordered military-trained thugs, known as the Falcons, to attack student marchers in Mexico City while he was president in 1971. At least 25 protesters were killed in the crackdown.

But the judge, Julio César Flores, threw out the charges, arguing that the 82-year-old Echeverría cannot be bound to a massacre that occurred 33 years ago given Mexico's 30-year statute of limitations.

Trusting Mexico's antiquated and corrupt judicial system with high-level cases like Echeverría's was a miscalculation, say analysts. In a February statement, New York-based Human Rights Watch warned Fox that if the special prosecutor's office did not receive more money and administrative support, its efforts could buckle under the weight of the dirty-war caseload.

Meanwhile, the Fox administration has decided against recommendations from local and international human rights groups to create a truth commission to examine past abuses. Other Latin American countries, like Argentina, Chile, Peru, and Guatemala, all hit with heavy doses of violent state-sponsored political repression during the cold war, established truth commissions that Human Rights Watch claims "have played a crucial role in improving the judiciary's capacity to handle human rights cases."

Government officials close to the special prosecutor's office say Fox backed away from the truth commission idea, seeing it as too controversial for the PRI to support. This has left much up to Carrillo, who says he will appeal Saturday's decision this week.

His legal tactic for going after the ex-president, along with 11 other high-ranking officials, was to charge them with genocide in the 1971 massacre. Some legal experts say that the genocide count, defined by Mexican law as "systematic crimes against the lives of members of any national group," is an attempt to catapult the special prosecutor's cases to Mexico's Supreme Court or the Inter-American Court of Human Rights, based in Costa Rica.

But the tactic could backfire if the evidence against Echeverría and others falls short of genocide. "This case is about a group of demonstrators and protesters and members of a left-wing group," says George Grayson, a Mexico scholar at William & Mary College in Williamsburg, Va. "What happened in 1971 in Mexico is not comparable with the Holocaust."

Kate Doyle, a senior Mexico analyst for the National Security Archive, a private research institute based in Washington, questions the prosecutor's approach. "The decision to base this case on genocide seems to be a very dangerous legal style," she says. "It can be called wildly ambitious or downright irresponsible."

However, Ulises Beltrán, a political scientist at the Center for Economic Research and Teaching, a Mexico City think tank, sees the legal wrangling as a sign of the country's democratic transition. "We have to respect the rule of law, even if we're not happy with the outcome," he says. "The legal system must run its course."

Joel Ortega, a Mexican human rights activist who witnessed the 1971 massacre, says: "Mexico has made a big deal about ratifying every international human rights accord out there. Why don't we follow the same codes at home?"

With barely two years left in Fox's term, it is doubtful that his legacy will include a meaningful correction of Mexico's faulty human rights record as long as PRI politicians control Congress. The president needs the PRI if he is to advance any of his reforms, and PRI politicians threaten to block Fox's every legislative move if the special prosecutor continues his work.

Polls show that Mexicans support punishing those responsible for the abuses of human rights during the dirty war. But some wonder whether Fox is slaying the wrong dragon. "Is it better to go after a tired, sick 82-year-old ex-president or should the energy be focused on solving the murders of women in Juarez, or the drug cartels, or crime that is taking place today?" asks Mr. Grayson.