LONDON — British spies hunted in vain for the creator of a fake recording of an alleged spat between British Prime Minister Margaret Thatcher and U.S. President Ronald Reagan, declassified documents revealed Friday.

Soviet spies, Argentine agents and British leftists were considered as possible pranksters, but an anarchist punk band later claimed responsibility.

The tape, sent anonymously to Dutch newspapers in 1983, purported to capture the two leaders sparring during the 1982 Falklands War between Britain and Argentina. A transcript shows Reagan urging Thatcher “to control yourself” and the British leader responding: “We have to use violence” to “punish” Argentina.

British authorities quickly identified the recording as a forgery, compiled from clips of the two leaders speaking in earlier interviews.

Thatcher’s press secretary, Bernard Ingham, wrote to another aide that the Reagan section was “far too fluent and articulate for the man in ordinary animated conversation.”

The hunt for the perpetrator appears to have been inconclusive.

A letter to Thatcher from a Foreign Office adviser said the MI6 intelligence agency had considered Soviet spies, Argentine intelligence agencies and left-wing groups as possible culprits.

But he said KGB involvement was considered unlikely, and an intelligence official concluded that “there is no information to indicate that any subversive group or individual in this country was involved in making this tape.”

The British punk band Crass later said it had created the hoax in a bid to sway opinion against Thatcher during Britain’s 1983 election. Thatcher won the vote in a landslide.

The papers were released Friday by Britain’s National Archives under a policy that declassifies many government papers after 30 years.