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General Augusto Pinochet directly ordered the 1976 assassination of a Chilean diplomat who was killed in a car bomb in Washington DC, according top secret US intelligence documents declassified by the Obama administration.

The documents, which were handed to the Chilean president, Michelle Bachelet, on Tuesday in Santiago by the US secretary of state, John Kerry, also show that the former dictator was so concerned with covering up his role in the murder that he planned to assassinate his own head of intelligence, General Manuel Contreras.

Orlando Letelier, a former defence and foreign minister under President Salvador Allende, was tortured and incarcerated after Pinochet’s 1973 coup. He later fled to the US and worked at the Institute of Policy Studies in Washington DC.

Letelier, who had once been Chile’s ambassador to the US, was murdered on 21 September 1976 by a car bomb planted under the driver’s seat of his vehicle just a mile from the White House.

Ronni Moffitt, an American colleague, was also killed in the blast. Her husband Michael survived but was badly wounded.

Letelier’s son, Senator Juan Pablo Letelier, confirmed to the Guardian that he had received copies of the newly released documents, which are understood to include papers from the CIA.

Letelier described reading the declassified documents and discovering a memo from George Shultz, who served as secretary of state in the 1980s, to President Ronald Reagan.

“[Shultz] informs [the president] that there is a conclusive document from the CIA that shows Pinochet ordered the murder of my father. This is concrete information about how Pinochet covered up his responsibility,” he said.

According to Letelier, who is among the first to have read the newly released documents, they also included evidence that the dictator intended to have his own spy chief murdered to cover up his role in the assassination.

“In his [Pinochet’s] predisposition to defend his position he planned to eliminate Manuel Contreras to keep him from talking,” said Senator Letelier in an interview with the Mesa Central programme on Tele13 Radio. Asked to clarify, Letelier said “physically eliminate”.

Peter Kornbluh, author of The Pinochet File: A Declassified Dossier of Atrocity and Accountability, said the documents “help provide historical accountability in the killing of these two wonderful people” and called them “the missing documents” in the ongoing efforts to unravel one of the most notorious acts of international terrorism in the US capital.

Kornbluh, a senior analyst at the National Security Archives, called for the full release of all the intelligence documents on Pinochet’s role in the Letelier-Moffitt bombing.

Investigators in the US and Chile are poring through the records searching for evidence that CIA officials had forewarning but did not stop the assassination plan.

Speculation that the CIA was aware of the plot to kill Letelier is based on previously declassified records showing that Manuel Contreras was paid by the CIA before the bombing and was in regular contact with top officials at the spy agency.

The US eventually sought to extradite Contreras of the murder, but Chile’s supreme court blocked the extradition.

Pinochet removed Contreras from his post under US pressure and dismantled and replaced the Dina spy agency he had once run. After Chile returned to democracy in 1990 Contreras was indicted in the Letelier case and eventually served seven years for the assassination.

Contreras, who died in August, always denied responsibility and blamed the CIA for the bombing

“It is very important that these documents have been declassified,” said the Chilean foreign minister, Heraldo Muñoz. “It helps us to clarify a painful historical moment for our country.”