Sebastian Pinera said his Interior Ministry will join and support an independent judicial probe of the 1982 death of Eduardo Frei Montalva, a former president and prominent Pinochet critic who died suspiciously after a routine hernia operation.
Six people, including doctors and former Pinochet spies, were charged in December 2009 with conspiring to poison Frei Montalva.
A secret U.S. Embassy cable published by WikiLeaks Monday predicted that questions about the death will never be fully resolved because evidence was destroyed when the dead president's body was hung from a ladder and drained of fluid, with key organs removed.
The Dec. 11, 2009, cable - an analysis of the case sent to Washington after the charges were filed - also says Pinochet's spies were known to have been developing chemical and biological agents to use against political enemies.
Pinera said that such dark episodes in Chilean history "must not remain in the shadows, that once and for all, the circumstances and those responsible should be made clear and that those who have responsibility assume the consequences."
He said his government owes this to the family of Frei Montalva, whose son he beat to become president last year, and that resolving questions about the death "is important to the interests of the entire country."
Frei preceded Salvador Allende as Chile's president, and at the time of his death at age 71, he was investigating human rights violations by Pinochet, who began his 17-year dictatorship by ousting Allende in a 1973 coup. Pinera's election returned the same right-wing parties to the presidential palace for the first time since Pinochet's rule.
An official autopsy report blamed the death on septic shock after stomach hernia surgery. But investigative Judge Alejandro Madrid said in 2009 that he found evidence showing Pinochet's intelligence agents covered up the real cause.
Madrid also credited a later autopsy by University of Chile pathologists with identifying two chemicals in Frei Montalva's body that attack the digestive system - one used in mustard gas and another found in rat poison.
"He was injected with toxic substances, which produced other complications that deteriorated his immune system. That was the cause of death," Madrid said.
Michelle Bachelet, the outgoing president, said "This goes to show that though it may take a while, justice in Chile always prevails."
The U.S. Embassy, however, said the "unofficial" autopsies done without support by the presidency had failed to preserve the chain of custody over evidence, and concluded that the full truth may never emerge because traces of the chemicals would have disappeared long ago.
"The Embassy is not aware of any direct evidence indicating foul play," the cable said. "Given the extremely long time since Frei's death and the destruction of some key organs, forensic science may not be able to provide definitive evidence whether Frei was murdered. Chile's tragic recent history continues to divide its people, and the death of this emblematic president seems destined to be yet one more area in which the full truth may never be known."
Phone and e-mail messages requesting comment from the U.S. Embassy in Santiago Tuesday were not immediately returned.