The family of former Chilean President Eduardo Frei Montalva says new evidence shows that he was murdered under General Augusto Pinochet's regime.
Former President Eduardo Frei Ruiz-Tagle is now a senator
During commemorations marking 25 years since Mr Frei Montalva's death, his son, Eduardo Frei Ruiz-Tagle, said the family would file a lawsuit soon.
Their actions come after Belgian researchers said they had found mustard gas in the former president's body.
Mr Frei Montalva was president from 1964 to 1970, and died in 1982.
His death in hospital was officially blamed on septicaemia following a stomach illness, but his family say they always suspected he was murdered.
"We have always said we want to know the truth and this is a substantial advance," said Mr Frei Ruiz-Tagle, who was himself president from 1994-2000 and is now a senator.
The family now wants the case to be reclassified and a murder investigation opened.