RIO DE JANEIRO  A judge in Santiago ruled Monday that a former Chilean president, Eduardo Frei Montalva, had been poisoned and charged three people connected with the Pinochet dictatorship with murder in the 28-year-old case.

Alejandro Madrid, a judge with the Court of Appeals, said there was evidence that Mr. Frei, who was president of Chile from 1964 to 1970, was poisoned with low doses of mustard gas and thallium in the months before his death on Jan. 22, 1982.

The poisoning at the Santa María Clinic in Chile’s capital compromised Mr. Frei’s immune system, the indictment said, and made him too weak to survive surgery for a stomach ailment, which the original autopsy had ruled as the cause of death.

The indictment charged six people in connection with the killing. A doctor connected to Gen. Augusto Pinochet’s army, a former intelligence agent under the general and Mr. Frei’s driver were charged with murder. Two doctors who were alleged to have falsified the autopsy report were charged with covering up the killing, and a third was charged as an accomplice.