Mr Frei Montalva was president from 1964 to 1970
A Chilean judge has charged six people over the death in 1982 of the country's ex-President, Eduardo Frei Montalva.
The judge said there was now evidence that Mr Frei, a vocal critic of military leader Augusto Pinochet, had been poisoned in hospital.
The arrests come less than a week before Chile's presidential election, in which Mr Frei's son, Eduardo Frei Ruiz-Tagle, is a leading candidate.
The former president's family have long argued he was murdered.
Mr Frei Montalva died after undergoing routine surgery.
The six people charged in connection with the alleged killing include four doctors - two who were involved in the operation and two in the subsequent post mortem.
At the time, the authorities said the former leader had died of a bacterial infection.
"The death of the former president came about as a result of the gradual introduction into his system of unusual, toxic substances... which broke down his immune system," Judge Alejandro Madrid said as he announced the charges.
The former leader's son, Eduardo Frei Ruiz-Tagle, himself president from 1994 to 2000, is running in Sunday's presidential election as candidate of the governing centre-left coalition led by President Michelle Bachelet.
Mr Frei's main opponent, Sebastian Pinera, said he felt nothing but solidarity for Mr Frei and his family, and described the former president's death as an open wound on the national soul.
Gen Pinochet's rule lasted from 1973 to 1990. More than 3,000 people who opposed his rule were killed or disappeared during this period.