Haydée Gastelú, one of the founding members of the Mothers of Plaza de Mayo, died on Monday at the age of 94, according to a statement from Mothers of Plaza de Mayo – Founders. The cause of her death was not immediately made public.

Argentina’s Human Rights Secretariat described her as an “example of resistance, memory and unwavering struggle for Human Rights”.

Gastelú had been a member of the Mothers of Plaza de Mayo since the group’s first ever march on April 30, 1977. She took to the streets with her comrades after her son, Horacio, was kidnapped on August 7 1976. It was ultimately established that he had been killed in the 1976 Fatima massacre, an atrocity in which 30 detainees were murdered by the police and their bodies were blown up in the town of Fatima, to the north of Buenos Aires.

In 2008, former police officers Juan Carlos Lapuyole and Carlos Gallone were found guilty of kidnapping and murder and handed life sentences for their participation in the massacre. The judges said that their deeds constituted crimes against humanity. However, a third officer, Miguel Ángel Trimarchi, was acquitted by the court, prompting many in the audience to shout insults at the jury.

“Horacio was taken from you by the last civic-military-ecclesiastic dictatorship and we lost him forever,” Mothers of Plaza de Mayo’s statement said. “Only his recovered remains gave you some peace and perhaps even a glimpse of happiness.”

“Since that fatal day, you have been a firm and incessant seeker of justice and truth. Serene and wise, your great heart made you a companion of so many Mothers.”

Gastelú also had two other children, Alicia and Diego.

Gastelú’s passing comes weeks after the death of Hebe de Bonafini on 20 November, at the age of 93. De Bonafini was also a founding member of the Mothers of Plaza de Mayo, although she remained with the Mothers of Plaza de Mayo Association after the group split into the Association and the Founders in 1986.

In addition to these groups, Grandmothers (Abuelas) of Plaza de Mayo was founded to identify the families of around 500 newborn babies who were born in detention and given to the families of military officials, taken anonymously to orphanages, or even sold.