Horacio Zacarias Barrientos, a former collaborator with security forces who turned state's witness against them, was killed not far from the resort city of Acapulco in the Pacific coast state of Guerrero, an official in the attorney general's office said on Friday.
The Mexico City daily La Jornada said Barrientos was tortured before he was killed on Wednesday.
The leader of a local survivors' group said Barrientos was silenced by forces seeking to protect former military and police officials and that prosecutors had been negligent in failing to protect him.
"We believe all of the complainants, the family members, the witnesses are in danger," said Julio Mata, who knew Barrientos.
Barrientos' death came as the public spotlight focused increasingly on state repression that occurred under the long-ruling Institutional Revolutionary Party, or PRI.
"We believe all of the complainants, the family members, the witnesses are in danger"
leader of survivors group
Hundreds of leftists were killed or disappeared in a dirty war by security forces, although the oppression was not on the scale of the brutality under military governments in other Latin American nations.
Survivors say repression during the 1970s was particularly brutal in Guerrero with the rise of insurgent groups there.
A special prosecutor released a document this week describing how leftist rebels and their supporters were routinely detained in Guerrero during the 1970s, interrogated, forced to drink gasoline, set on fire and shot.
President Vicente Fox ousted the PRI in 2000 promising to punish past crimes in high places. He named a special prosecutor two years ago to investigate dirty war crimes.
First arrest order
On Wednesday, a federal court in Guerrero issued the first arrest order in a dirty war case with a summons for Isidro Galeana, a former state police commander accused of the 1974 kidnapping of a leftist teacher who was never seen again.
Galeana's son Gustavo told reporters in Acapulco his 68-year-old father was suffering from diabetes, liver and lung problems and was away from home this week receiving medical treatment.
Galeana had not been arrested by Friday.