Two former Mexican presidents are denying any involvement in the so-called "dirty war" against dissidents and leftist guerrillas that occurred during their terms of office in the 1970s. This follows the announcement earlier this week that a special prosecutor will be named to investigate the cases.

The dirty war of the 1970s has become front page news again here in Mexico as a result of a report by the nation's Human Rights Commission handed to President Vicente Fox on Tuesday. The report provides details on more than 500 cases including those of 275 persons who vanished without a trace. The Human Rights Commission regards those cases as "forced disappearances" and makes it clear in the report that the Mexican government was responsible.

For his part, President Fox said a special prosecutor will investigate the cases and those found responsible will be punished. Mr. Fox says former presidents and other high ranking officials will not be immune from this process.

But the two men who served as president during the period in question deny having any knowledge of the events. Former president Luis Echeverria, who served between 1970 and 1976, says he put his faith in the Mexican military.

He says he has faith that the Mexican military carried out its duties in a patriotic spirit and that if there were what he called "police excesses" that they should be punished.

Former President Jose Lopez Portillo, who assumed office in 1976 and left in 1982, told reporters he had no knowledge of any dirty war.

He says the fight against leftist guerrillas was something that occurred throughout Latin America during that time but that he never authorized extra-legal means of dealing with the problem. He says he always tried to act within the law.

Some of those who were persecuted during that time or who lost family members in the dirty war say the former leaders should be held responsible, along with other former officials including military chiefs. They say the strength of Mexico's new democracy can only be tested by clarifying the human rights abuses that occurred in the past.

Both Mr. Echeverria and Mr. Lopez Portillo were members of the Institutional Revolutionary Party, or PRI, that ruled Mexico for 71 years. That grip on power ended when Mr. Fox, of the National Action Party, won the July, 2000 presidential election, promising a more open government and an investigation into past government abuses.