Family members of at least five missing victims looked on, Mata told The Associated Press. The attorney general's office declined Sunday to provide information on the search.

Atoyac de Alvarez, a center of guerrilla activity in the 1960s, is the birthplace of one of the era's main rebel groups, the Party of the Poor, which was led by former school teacher Lucio Cabanas. Cabanas was killed during a gunbattle with authorities in 1974.

Several rebel groups carried out kidnappings and bank robberies in the 1960s and 1970s, prompting police and military crackdowns that led to the disappearance and deaths of an estimated 275 to 1,300 people.

"Some were taken by plane and thrown into the ocean," Jose Luis Arroyo, who has been searching for his nephew since 1972, said Saturday outside the former base. "Many never came back."

If investigators find evidence of human remains, they will unearth bodies to perform DNA tests, said Mata, executive secretary of the Mexican Association of Families of the Missing, Detained and Victims of Human Rights Violations. He previously vowed his group would file a complaint against the Mexican army if remains were found at the former base in Mexico's southern state of Guerrero.

"When we were detained, we would hear machines here at the military base digging," said Jose Luis Blanco, who was held at the base in the late 1970s and early 1980s. "The soldiers would threaten us, saying they buried the dead there."

Former President Vicente Fox, who governed from 2000 to 2006, created a special prosecutor's office to investigate crimes related to the killing of rebels.

But after a five-year investigation, the office's only achievement was to bring charges against former President Luis Echeverria for his alleged involvement in a Mexico City massacre of protesting students in 1968. A court exonerated Echeverria, who was interior minister at the time of the killings.