The declassified documents will inevitably lead to questions about the US government's own role in the coup against President Salvador Allende in September 1973 and the bloodbath that followed in which 3,000 were murdered or "disappeared".
"There are thousands of documents in the records of the CIA, the Pentagon, the State Department and elsewhere that would prove Spain's case against General Pinochet," said Peter Kornbluh, an analyst with the National Security Archive, the Washington based freedom of information group.
The decision could reveal the depth of US knowledge and involvement in the coup. There is speculation that it could result in international law suits against former US government officials.
One member of the British embassy staff at the time of Pinochet's coup was David Spedding who served in Chile from 1972 to 1974. Now Sir David, he is head of the Secret Intelligence Service, MI6.
The documents will back up the evidence that Dr Kissinger not only sought the liquidation of Allende, the freely elected left-winger, but also did nothing to stop the systematic use of torture by the Pinochet regime.
The US is also said to have done nothing to prevent the assassination by Pinochet's secret police, the Dina, of Orlando Letelier, Allende's former ambassador to the US, in Washington in September 1976. In June the State Department had learnt that the Dina had started an undercover operation in the US capital and that the government of Paraguay had sought US visas for two Dina officers.
George Landau, US ambassador to Paraguay, sought guidance about the two men from Washington, a request which was passed to Dr Kissinger's office and also to the CIA.
One already declassified document, written by the Assistant Secretary of State, Jack Kubisch, 10 weeks after the coup, shows that US intelligence knew that Pinochet's military had already executed 320 people, three times the officially admitted number, and carried out extensive human rights abuses. The memo goes on to say that the US government had already sent $24m of aid to the new regime.
US government and CIA officials who played a part in America's efforts to destabilise Allende's elected government could also come under scrutiny.
In front of Dr Kissinger, President Nixon told the CIA chief, Richard Helms, to do whatever it took to prevent Allende from winning office. "Make the economy scream," he said. He told them to organise a military coup and made $10m available.
Although the CIA tried, they did not manage to stop Allende in 1970. Helms was later successfully prosecuted for perjury for lying to Congress about the coup attempts. He is now retired.
CIA operations to destabilise Allende continued. Between 1970 and 1973 the US spent nearly $8m in covert operations. Nixon was motivated by his friends in big business, especially the ITT conglomerate which wanted Allende out.Reuse content