The documents were presented by U.S. Secretary of State John Kerry to Chile's President Michele Bachelet, whose father had died following his time in one of Pinochet's prisons and who herself lived through detention and exile.
"We have long known that the Chilean secret police/intelligence service was behind this brutal act, perhaps the only clear case of state-supported terrorism that has occurred in Washington, D.C.," Shultz wrote to Reagan in a now-declassified memo dated Oct. 6, 1987.
"It is not clear whether we can or would want to consider indicting Pinochet,” Shultz wrote. "Nevertheless, this is a blatant example of a chief of state's direct involvement in an act of state terrorism, one that is particularly disturbing both because it occurred in our capital and since his government is generally considered to be friendly."
Shultz added: "While some in the [U.S. government] had previously believed that Pinochet had ordered the murders, and there were strong signs that he was involved in the cover-up, the CIA has never before drawn and presented its conclusion that such strong evidence exists of his leadership role in this act of terrorism."
Senior figures within the State Department and the CIA were attempting to convince Reagan of the need to push for greater democracy in Chile. Despite his hideous record of rights abuses, Pinochet remained an ally of the West's inveterate Cold Warriors, including Reagan and British Prime Minister Margaret Thatcher.
"This document is clearly the holy grail of the case," said Peter Kornbluh, author of "The Pinochet File: A Declassified Dossier of Atrocity and Accountability" and director of the National Security Archive's Chile project at George Washington who directs the Archive’s Chile Documentation Project. In a statement, he called on the CIA to release further information of what it knew of Pinochet's operations, and particularly the Letelier assassination.
Letelier, who had served as a senior minister in Allende's government, was killed Sept. 21, 1976, when a bomb in his car exploded on Massachusetts Avenue while he was en route to his job at a Washington think tank. A 25-year-old colleague accompanying him also died.
Pinochet eventually stepped down in 1990 after a popular referendum rejected his continuing in power, paving the way for a more democratic Chile. At the time of his death in 2006, a majority of Chileans believed he should be prosecuted for human rights crimes.