A special prosecutor appointed by President Vicente Fox said he would decide in the next 10 days whether to bring criminal charges against former president Luis Echeverria involving a 1971 massacre in which security forces killed at least 30 young protesters in Mexico City.

Prosecution of Echeverria, 82 -- whose tenure from 1970 to 1976 was at the height of what became known as the government's dirty war against pro-democracy activists -- would set up a showdown between Fox and the political party that ruled Mexico for most of the 20th century.

Officials from the Institutional Revolutionary Party, or the PRI, said this week that they would regard charges against Echeverria as a provocation that could rupture the party's already tenuous relations with Fox, whose election in 2000 ended the PRI's 71-year lock on power. During that era, a succession of PRI presidents ruled with impunity, then were assured a quiet retirement free from scrutiny.

A statement issued Monday by the PRI said charging Echeverria would be "an attack on our institutions."

"This is a political act, not a legal one," said David Penchyna, secretary of the PRI's national council. "This administration is stuck in the past as opposed to looking toward the future. Looking at the past doesn't get us anywhere."

Fox has pledged that Mexico must have an accounting of crimes committed by past governments and that not even former presidents would be immune. He has said he would not interfere with the investigation by the special prosecutor, Ignacio Carrillo Prieto.

Echeverria, who declined through his attorneys to comment for this article, told reporters outside his house that he was "relaxed" and believed that Carrillo Prieto had no case against him. His attorney, Juan Velazquez, said in a televised interview Tuesday that "there is no proof, absolutely no proof" of Echeverria's involvement in the 1971 massacre.

But for many Mexicans and human rights activists, Echeverria is a symbol of the worst violence of the dirty war. He was secretary of interior, the second most powerful post in government, during the Oct. 2, 1968, massacre in Mexico City's Tlatelolco Plaza, in which Mexican police and soldiers opened fire on anti-government protesters and killed an estimated 300 people. Carrillo Prieto's office is also investigating Echeverria's role in that incident.

Former student leader Enrique Condes Lara, a historian who has written books about the dirty war period, said there is ample documentary evidence that Echeverria directed government agents on June 10, 1971, during what became known as the Corpus Cristi massacre.

"The documents show that he was passed reports every 10 minutes while everything was happening," said Condes Lara, who said he gave the documents, found in Mexico's recently opened national archives, to Carillo Prieto's investigator.

Condes Lara said Echeverria oversaw the actions of the Falcons, a secret force trained and paid by the government that allegedly carried out the killings that day. He said Echeverria was also ultimately responsible for a wave of violent repression and the killings of hundreds of anti-government activists, many of them farmers, students and teachers, during the 1960s and 1970s.

Charges against Echeverria "would be a very important step toward building a country of rule by laws, not by political vengeance or by abuse of authority," Condes Lara said. "To build a democratic state, you must first settle your accounts from the past."

Salvador Martinez Della Rocca, a federal congressman from the opposition Democratic Revolutionary Party, was present at the 1971 protest in northern Mexico City. He said he ran and escaped when the shooting started, but several of his friends were killed.

"It was a peaceful, legal march of students and for that we were savagely massacred," Martinez said. "Echeverria was either responsible, or he knew about it and didn't do anything, which also makes him responsible. . . . He was president during some of the worst moments in Mexican history."

Rosario Ibarra, a prominent human rights activist whose son, Jesus Piedra Ibarra, was a student activist who disappeared in 1975, said Mexico's military and law enforcement institutions appear to be rallying around Echeverria.

Ibarra said she feared that the former president would never be punished. "They always promise things that don't happen," she said.

Defense Minister Gerardo Vega Garcia recently suggested that, in the name of national reconciliation, the government should drop investigations of dirty war crimes -- many of which were carried out by soldiers. The country's top organized crime prosecutor, Jose Luis Vasconcelos, also recently suggested that police and soldiers acting long ago and under orders should be pardoned, telling Mexican reporters, "It's hard to pass judgment on someone for something that happened under different circumstances."

Luis Echeverria, Mexico's president from 1970 to 1976, is the focus of a probe into government repression.