A Mexican judge yesterday refused a special prosecutor's request for an arrest warrant against former President Luis Echeverria and several other former top officials in connection with a 1971 massacre, a significant setback in a case widely viewed as a test of whether Mexico will come to terms with abuses committed by the country's former authoritarian leaders.

Officials in the prosecutor's office said the judge ruled that Mexico's 30-year statute of limitations for genocide had expired in the 1971 killings for which Echeverria had been accused. The prosecutor is seeking to charge the 82-year-old former president, who was in office from 1970 to 1976, with the killings of about 30 student protesters by government-backed security forces in Mexico City in June 1971.

Judge Jose Cesar Flores Rodriguez did not make public the specific legal reasoning for his decision, as is common practice in Mexico. Special prosecutor Ignacio Carrillo Prieto read a statement on national radio last night vowing to appeal the decision to the Mexican Supreme Court within three days.

Echeverria's lawyer, Juan Velazquez, said in a radio interview yesterday that the judge's ruling was "just."

The killings constituted one of the most notorious episodes in the Mexican government's violent campaign against anti-government activists from the 1960s to the 1980s, known as the dirty war. Investigating that era of Mexican history has been a source of friction between the Institutional Revolutionary Party, or PRI, which held power in Mexico from 1929 until 2000, and President Vicente Fox, who unseated the PRI in part on a pledge to come to terms with the official abuses of the country's past.

The news that broke Friday that the special prosecutor was filing charges and seeking an arrest warrant for Echeverria raised expectations among the news media and the public that one of the most reviled figures in Mexican politics was finally going to be held accountable. Some Mexican officials publicly reacted as if acceptance of the charges were already a fait accompli.

But in the Mexican judicial system, where there are no grand juries or jury trials, prosecutors present evidence to a judge, who evaluates it and decides whether to issue formal charges and arrest warrants. In this case, the judge turned down the prosecutors' request for a warrant.

That decision reinforced the feeling among many Mexicans that while Carrillo Prieto was gaining ground in his attempt to clarify the past, the Mexican political and judicial systems were not ready for an aggressive investigation into a disturbing chapter in Mexico's history.

"People in power are always going to help other people in power," said Agustin Cortina, 21, an architecture student from Guadalajara. "It's all so dirty, it's never going to change. As much as someone might try to change things, it's impossible."

"Things haven't changed, they've just gotten worse," said Josefina Zarco, 61, a housewife from Chiapas state. "President Fox says he's going to change things, but he hasn't."

Carrillo Prieto argued that the statute of limitations had not expired. He said government investigations into the 1971 killings, which were known as the Corpus Christi massacre, continued until at least 1982, and that Mexican law stipulates that the clock for the statute of limitations starts at the end of the last investigations into a crime.

Carrillo Prieto said he trusted the Supreme Court to rule in his favor, citing recent court rulings that were "essential foundations of a democratic rule of law."

He was apparently referring to the court's ruling last year that the statute of limitations had not expired in cases of officials implicated in the forced disappearances of people whose bodies had never been found. The court ruled that those were ongoing cases because the bodies had not been recovered.

That ruling was a major victory for Carrillo Prieto in his effort to seek arrest warrants against officials allegedly involved in disappearances and killings in the 1960s and 1970s.

Laurie Freeman, a Mexico specialist at the Washington Office on Latin America, a policy and rights group, said that yesterday's ruling was not surprising because "the case is so political."

"But the special prosecutor's arguments are compelling, so I think he still has a chance before the Supreme Court," Freeman said.

In his decision, Flores apparently did not discuss the merits of the genocide charge. Analysts in Mexico and the United States have said they do not understand how Carrillo Prieto would prove that Echeverria committed genocide, defined in Mexican and international law as crimes against people of a particular race, nationality or ethnicity. Carrillo Prieto has not made public all the details of his arguments against Echeverria.

But even some of Echeverria's staunchest critics, who believe he is responsible for many crimes, wonder if Carrillo Prieto, in the face of enormous political pressure and legal complexities, is reaching too far in his attempt to bring genocide charges.

"Was Kent State genocide?" said Adolfo Aguilar Zinser, Fox's former national security adviser and former ambassador to the United Nations, referring to the killing of four students in 1970at that Ohio university.

Judge Flores was involved in a similar high-profile case in 2001. Carlos Cabal Peniche, a fugitive banker who had allegedly bilked investors out of millions of dollars, was arrested in Australia after three years of hiding and extradited to Mexico in September 2001.

According to Mexican media reports, Cabal Peniche was brought before Flores, who freed him, ruling that the statute of limitations had expired for the crime with which Cabal Peniche had been charged. The attorney general's office appealed and Flores's decision was overturned, news media reports said. The Cabal Peniche case was returned to Flores, who imposed a fine of more than $400 million in 2002, the reports said.

Eric Olson, spokesman for Amnesty International in Washington, called Flores's decision in the Echeverria case disappointing. "We hope it's not the final word in the search for justice in these cases," he said.

Researchers Gabriela Martinez in Mexico City and Bart Beeson in San Miguel de Allende, Mexico, contributed to this report.

Former President Luis Echeverria, shown in 2002, escaped charges.