BY ANDRES OPPENHEIMER

  MEXICO CITY -- Former President Luis Echeverría, a populist who governed Mexico from 1970 to 1976, has emerged as the principal target of an unprecedented
  government investigation into the long-hidden ''dirty war'' waged against anti-government activists decades ago.

  As if to emphasize the government's determination to punish wrongdoers, top officials say they will invoke international treaties to overcome a 30-year statute of
  limitations law that would prevent the prosecution of Echeverría for gross human rights abuses in the 1970s.

  Interior Minister Santiago Creel said the 80-year-old Echeverría is being investigated for his alleged role in the 1968 and 1971 killings of dozens -- perhaps hundreds --
  of leftist activists, many of whom disappeared after clashes with government forces in Mexico City demonstrations. The first incident took place while Echeverría was
  interior minister under his predecessor, the late Gustavo Díaz Ordaz.

  The Echeverría probe is among half a dozen investigations that are putting to a test a vow by President Vicente Fox to end Mexico's tradition of allowing powerful
  politicians to get away with commiting crimes. Leaders of the party once led by Echeverría claim Fox has embarked on a witch hunt, and threaten to strike back with
  labor union strikes and other protests if the government goes ahead with the inquiries. ''In cases of disappearances, we will take the position that this is a continuing
  crime,'' Creel said in an interview at his office. ``There will be a debate about this, but we believe there is a basis to make a good argument that the statute of limitation
  on these crimes has not expired.''

  Echeverría, who championed Third World causes and used to lash out against the United States during his presidency, had long been the target of press allegations that
  he had authorized the killings. But it wasn't until President Fox's victory in 2000, when Echeverría's Institutional Revolutionary Party (PRI) lost power after seven decades
  of often authoritarian rule, that the government started a serious investigation into the case.

  Last month, the Mexican government declassified millions of government files on the ''dirty war'' of the 70s and 80s. In addition, Fox appointed a special prosecutor to
  look into the 532 documented cases of political killings and disappearances, including those in the 1968 clash at Mexico City's Tlatelolco Square and the 1971 student
  demonstration in the city's San Cosme district.

  Until recently, Echeverría lived the privileged life of an elder statesman of the PRI. He was a frequent visitor at the presidential residence of Los Pinos, and
  recommended loyalists to powerful jobs. One of his children, Benito Echeverría, has headed a Mexican government tourism office in Miami for about seven years.

  MANY QUESTIONS

  But, in a scene reminiscent of Chile's recent probes into former dictator Gen. Augusto Pinochet, Echeverría was summoned by special prosecutor Ignacio Carrillo Prieto
  to testify last month and was greeted with shouts of ''assassin!'' by scores of demonstrators. The prosecutor gave the former president more than 150 questions, which
  he will have two months to answer.

  Last week, Echeverría was hospitalized, suffering from non-life-threatening respiratory infections, the health ministry said. In a statement, the ministry said the
  ex-president was admitted to Mexico City's Ignacio Chavez National Cardiology Center on Thursday for routine tests. Echeverría's doctors said he was no longer
  suffering from fever, that his prognosis was good and that they expected him to be out of the hospital ``in a few days.''

  Government prosecutors want to know whether Echeverría -- as many of the victims' relatives say -- ordered the repression of the 1968 student demonstration, where
  army troops killed at least 30 leftist activists, while he was interior minister. Some historians believe many more were killed in that incident. Echeverría has said in the
  past that the orders had come from President Díaz Ordaz, and that the late president himself had publicly admitted that.

  But the main charges against Echeverría focus on the June 10, 1971, killings in San Cosme by a para-military group known as ''Los Halcones'' (The Falcons), reportedly
  created by Díaz Ordaz and assigned to patrol the streets and subway stations.

  `TWO BIRDS'

  Former Mexico City Mayor Alfonso Martínez Domínguez was quoted in a 1979 interview with the weekly Proceso as saying that the killings by the paramilitary group
  ``were engineered by Luis Echeverría to kill two birds with a stone: He wanted to scare those who he said were trying to harass his government at the very start of his
  term, and he got rid of me.''

  According to human rights groups, the paramilitary group not only shot at the demonstrators, but also went to several hospitals afterwards to kill survivors in the
  emergency wards.

  Echeverría declined requests for an interview, but his top attorney, Juan Velázquez, disputed the critics' version of the events.

  Velázquez said the Falcons were ordered by ''somebody'' to crack down on the demonstrators with canes and, when they were met with gunfire from the demonstrators,
  went back to their headquarters to get weapons. The Falcons did indeed go the hospitals later that day, but it was to take their own wounded, the attorney said.

  At any rate, prosecution for both the 1968 and 1971 crimes has been proscribed, and the government will not be able to invoke international conventions against
  genocide to press charges against Echeverría, Velázquez said.

  ''Assuming, without conceding, that there was a genocide, the 30 years the law would have required to file charges has already expired,'' Velázquez said. ``And
  Mexico's Supreme Court ruled in 1999 that, when there is a question of hierarchy between the Mexican Constitution and international conventions, the Constitution
  comes first.''

  LATER CRIMES

  Some academics concede that the Supreme Court ruling may well protect Echeverría for these two incidents. But they add that the former president could still be
  prosecuted for later crimes, and will at any rate end his life shrouded in controversy.

  ''It may be too late to do justice in connection with the events of 1968 and 1971, but there can still be legal action regarding actions that took place after 1973, which
  have not prescribed,'' says leading historian Lorenzo Meyer. ``At any rate, forcing Echeverría to respond for his actions sets an important precedent to prevent these
  crimes from remaining unprosecuted in the future.''