THE CIA obtained “excellent samples” of airborne debris from China’s first nuclear test explosion in a cold war espionage coup that was confirmed for the first time last week.

A specially fitted British Canberra bomber flying out of Hong Kong and a team of Tibetan exiles trained as CIA agents may also have played a part in one of the most successful operations ever staged against a communist power.

The CIA has disclosed the extent of its spying on China’s nuclear programme in 71 documents declassified under the Freedom of Information Act. The papers also reveal a frightening lack of knowledge about who controlled China’s nuclear arsenal and how it might have been used.

The Chinese nuclear programme was an important theme in US national intelligence estimates from 1948 and 1976. An assessment of August 26, 1964 gave a warning that “on the basis of new overhead photography” the CIA believed China was completing a test site at Lop Nor in the western deserts of Xinjiang province.

John McCone, the CIA’s director, took the information to London where he found a ready audience. Britain had already told the Americans of confirmation from Mao Tse-tung himself that the Chinese were trying to build the bomb.

“We are preparing to make some,” Mao told Viscount Montgomery of Alamein, who called on him in Beijing. “It is something to scare people, absorbing a lot of money but useless.”

Montgomery alerted intelligence officials in Britain, where the authorities swung into action alongside the CIA.

The joint intelligence committee (JIC) ordered a specially fitted Canberra bomber to be sent to Hong Kong and offered to cover air sampling of an arc of territory between Hong Kong and Singapore, according to Richard Aldrich, the historian.

“Our chances of getting the fullest available information from the Americans would be very much helped if we took a hand in the information collecting effort,” read a Ministry of Defence document quoted by Aldrich in a recent book.

At the same time a CIA operation was in progress to drop Tibetan exiles as spies and saboteurs into their mountain homeland, which was already under Chinese occupation. One of their tasks was to plant remote sensors on the plateau south and east of Lop Nor.

On October 16, 1964 the Chinese detonated their first nuclear device, heralded by a blast of triumphant propaganda. However, Mao would have been disconcerted to learn that by January 27, 1965 the CIA was able to tell President Lyndon Johnson almost everything about his bomb.

“Our analysis of excellent samples of the test debris indicates that the Chinese communists’ first nuclear detonation was a well prepared scientific experiment,” read a top-secret national intelligence estimate.

It identified the bomb as a Hiroshima-type implosion device using uranium 235. There is no clue in the published CIA documents, sections of which remain blacked out for security reasons, of how the agency obtained the samples.

Evidence now available elsewhere, however, suggests that both the British and Tibetans played their part in capturing the tell-tale particles.

China’s nuclear operations were to preoccupy American and British agents in Hong Kong through the chaotic years of Mao’s cultural revolution and after his death.

Yet by the 1970s, as Richard Nixon prepared for a historic trip to China, the CIA had to admit that while it knew a lot about Chinese nuclear missiles and bombs, it had almost no idea of how and when they would be deployed.

“They may not have developed much doctrine beyond the conviction that possession of such weapons was essential if China were to join the ranks of the leading military powers,” read an assessment sent to Nixon in 1971. “We have no way of knowing.”

The Americans were confident, however, that China had no wish to start a nuclear war. Its small force was essentially a deterrent, the CIA told Nixon, but “we do not know how the Chinese would proceed should deterrence fail”.

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