The U.S. government blamed the KGB; British intelligence agencies pointed the finger at Argentine spies. They couldn’t have been further from the truth.

The full story of a hoax recording of Ronald Reagan and Margaret Thatcher can now be disclosed after a raft of secret documents were declassified in London. A crackly tape purporting to capture a confrontational phone call between Reagan and Thatcher in 1982 was created as a prank in the Essex bedroom of an English punk band, and yet we now know it was being analyzed by the British Prime Minister and top officials in Washington years later.

The tape, which was delivered anonymously to the offices of a Dutch newspaper, apparently recorded a serious disagreement between the leaders of the “free world” over Britain’s controversial conflict with Argentina in the Falkland Islands.

An enclosed transcript claimed the president told the Iron Lady “to control yourself” and upbraided her for sanctioning the sinking of an Argentine ship as it sailed away from the disputed islands, killing 323 people. “Why was the Belgrano destroyed? You gave the orders to do it. The Argentines were leaving at the time,” Reagan supposedly said.

The editor of the NRC Handelsblad newspaper took the tape to London and played it for Sir Bernhard Ingham, Thatcher’s press secretary and one of her closest Downing Street aides. According to classified documents released on Friday, Ingham sought the counsel of other officials before telling the Dutch journalist “the conversation was a total fake.” “I regarded the whole thing as a fabrication, a hoax, a put up job,” he wrote in May 1983.

But we now know the story didn’t end there. It was still being discussed at the highest levels of government more than a year after that meeting. The finest minds in the Western intelligence community were flummoxed.

Downing Street began examining the records of Thatcher’s conversations with the president in search of the contentious call. Copies of the tape were seized by the British and American embassies in The Hague who set their intelligence specialists to work on discovering the source of the recording. Both soon agreed that these were the voices of Thatcher and Reagan but they had been spliced together to create the hoax recording.

It’s “a rather clumsy operation” concluded one British intelligence official. “For their part, the Americans are attempting to make a similar check. Their preliminary technical analysis supports the conclusion that it is a clear fabrication,” he wrote, suggesting it was below the standard one would expect from Soviet disinformation specialists. “It is possible that one of the Argentine intelligence services might have been behind it,” he concluded.

Another top secret note from the Foreign Office said: “There is no information to indicate that any subversive group or individual in this country was involved in making this tape.”

About six months later, a U.S. State Department official reportedly suggested the Soviet Union might be to blame: “This type of activity fits the pattern of fabrications circulated by the Soviet KGB.”

But, it wasn’t Argentine espionage agents or Moscow plotting to destabilize Thatcher ahead of the 1983 election. The tape was manufactured by a little-heard punk band called Crass. “They were not terribly accessible even to punk or alternative listeners,” said Ritchie Unterberger, author of Unknown Legends of Rock’n‘Roll. “They were the most uncompromising punk band from anywhere.”

The anarchic band had cut the tape partly as a prank, partly to raise questions about Thatcher’s pro-nuclear, pro-war views. But it wasn’t the intelligence agencies that uncovered their part in the diplomatic incident, it was a British newspaper. The Observer outed the band behind Penis Envy and Christ the Album as the culprits in January 1984.

Three months later, Thatcher asked the Home Secretary and the Foreign Secretary whether she could mention the tape during an upcoming BBC television interview. The response came back quoting conflicting press reports that blamed both the KGB and Essex’s premier anarcho-punk band. The advice was: best to leave it well alone.

After the publication of the documents this week, Steve Ignorant, Crass’s lead singer, said it was incredible that their hoax had caused such high-level discomfort for so long. “It makes me a bit worried about governments because if they could be fooled by something so ridiculous… well, someone told us that there was an MI5 dossier on us but we didn’t take it that seriously,” he said.

Most of the members of the band remain reluctant to speak to the press 30 years after their greatest prank. The official website of Crass co-founder Penny Rimbaud features nothing but a short video of the gnarled rocker exhaling a lungful of smoke and muttering two words: “Shit happened.”