MEXICO CITY -- Luis Echeverria, a former president of Mexico, has broken a pledge to testify about the killing of student protesters by government forces in the 1960s and 1970s and has reverted to the stony silence he has kept for three decades.
Echeverria refuses to respond to a special prosecutor investigating the deaths. His reversal, made last week through his attorneys, sets up a constitutional question without precedent in Mexico: Does a former president have the right to remain silent when called to account for crimes of the state?
"His silence speaks volumes," said Ignacio Carrillo Prieto, the special prosecutor appointed by President Vicente Fox. "But it also stymies the investigation."
That leaves a dark chapter in Mexico's history unresolved.
Echeverria was president from 1970 to 1976, and before that was interior minister, Mexico's most powerful internal security official. Hundreds of suspected enemies of the state were killed or tortured or disappeared during those years.
Historians and veterans of the student movement hold Echeverria responsible for two massacres in Mexico City, one in 1968 during a protest at Tlatelolco Plaza, the second in 1971 during a march on the city's central square. Though the number of dead is disputed, scores of people, possibly hundreds, died at Tlatelolco. At least 29 were killed in the 1971 march.
Last month, in an event unimaginable in years past, Echeverria was called to testify for his acts in office. After a long interrogation during which he sat in silence, he received a list of 186 written questions from Carrillo. Antonio Cuellar, an attorney for Echeverria, said that the former president would answer within 40 days.
"He has nothing to hide," Cuellar said. "Our strategy is to testify, and truthfully."
That strategy has changed. Last week, through his attorneys, Echeverria invoked his right to remain silent.
Mexico's Constitution protects a defendant against self-incrimination. Echeverria says he will not testify against himself.
Carrillo has the power to try former government officials for abuses of power, though. His appointment came after Mexico's Supreme Court ordered Fox's government to investigate the Tlatelolco killings, and Fox promised to seek the truth about the past.
Few nations have successfully prosecuted presidents. In the United States, a presidential pardon absolved Richard M. Nixon from any charge after he resigned rather than face impeachment in 1974. Bill Clinton, impeached by the House for perjury and obstruction of justice in 1998, was neither convicted by the Senate nor indicted after leaving office.
Carrillo has not said that Echeverria will be accused of any crime. Mexico's 30-year statute of limitations for crimes, including murder, makes it doubtful that the former president could be prosecuted for the 1968 or 1971 massacres.
But Carrillo has a long witness list of former government officials he plans to summon. Echeverria may be called once again to account for the past.