MEXICO CITY (Reuters) -- An important witness for prosecutors investigating atrocities by Mexican security forces in "a dirty war" against leftists 30 years ago was shot to death this week, officials said on Friday.

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.

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.

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.

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. "He (Barrientos) had made declarations to the prosecutor," Eduardo Maldonado, a spokesman for the prosecutor's office, said. "He was important (as a witness)."

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.

Survivors say repression during the 1970s was particularly brutal in Guerrero with the rise of insurgent groups there.

The 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.