Herbert Anaya, the head of El Salvador's nongovernmental Human Rights Commission who was assassinated Monday, was the fourth member of the group to be killed since it was founded in 1978. At least three others have disappeared and are presumed dead.

Anaya and other members of the group had reported numerous death threats in recent months.

The group, made up of lawyers and other volunteers, investigates cases of political killings and disappearances.

During its early days, its members did legal work for political prisoners and their families. At the height of the political killings in El Salvador in the early 1980s, its members would go to places where the bodies of death squad victims were dumped, photograph the bodies and report on their condition. It kept photographs of unidentified bodies for the friends and relatives of people who had disappeared.

The group has often been accused of being sympathetic to leftist guerrillas fighting to overthrow the government of President Jose Napleon Duarte. Anaya and several other members of the commission were arrested on suspicion of aiding the guerrillas in May of last year.

The arrests followed public statements in government-sponsored press conferences by a former member of the group who said she had turned over money raised by the group to the guerrillas.

While in Mariona prison, Anaya and other members of the group interviewed their fellow prisoners about their treatment and later wrote a report alleging that prisoners were abused there.

Anaya and other members of the group were freed in February in exchange for a Air Force officer captured by the guerrillas.

The human rights group Americas Watch has identified the other members of the group who have been killed as: Maria Magadalena Henriquez and Ramon Valladares, the press secretary and administrator of the group. They were abducted and murdered Oct. 3, 1980. Several witnesses said Henriquez was detained by police. Marianela Garcia Villas, president. She was investigating reports of human rights abuses in Suchitoto in March 1983 when refugees she was accompanying apparently were mistaken for guerrillas and ambushed by an Army patrol.

According to Americas Watch, those abducted are: Dr. Roberto Rivera Martelli, a founding member, seized in 1983; America Fernanda Perdomo, a staff member, in 1982, and Carlos Eduardo Vides, in 1981.