The findings come from the Swedish Defence Research Agency (FOI) which has employed new methods to once again analyze the 3 minutes and 47 seconds of tape recorded by the marines on October 12th, 1982.
The agency was asked to determine whether the sound on the recording originated from a submarine or surface vessel.
In the spring of 2008, FOI learned that the charter boat Amalia was in the area when the recording was made in 1982 and consequently made a recording of the Amalia in April of this year to compare to the original.
“The conclusion is that it was likely the charter boat Amalia, which was in the area the same day, which is the source of the cavitation sounds from a propeller found on the recording,” said a statement from FOI.
The 1982 Hårsfjärden incident was the result of Operation Notvarp, one of the most advanced secret submarine hunting operations ever undertaken by Sweden’s Armed Forces.
The operation concentrated all of Sweden’s submarine hunting forces in one location following a number of suspected incursions by Soviet submarines into Swedish waters.
Characterized as “submarine fishing”, the operation involved trapping a foreign submarine and forcing it to the surface by dropping depth charges.
Following the operation, a special submarine commission was launched which in the spring of 1983 concluded that the Soviet Union was behind the incursions.
“There are several recordings from that time, but this particular tape was kept secret until quite recently,” FOI spokesperson Johan Axell told The Local.
“It is the only recording to remain classified for so long and was presented as hard evidence against the Russians.”
The sitting Swedish government at the time recalled Sweden’s ambassador to Moscow and registered an uncharacteristically strong protest to the Soviet government.
The centre-right opposition backed up the government.
“The Soviet Union is challenging Swedish neutrality. We cannot accept that,” said Liberal Party leader Ola Ullsten at the time.
The Soviet government in turn protested against Sweden’s complaint and rejected all assertions of incursions into Swedish waters.
According to Axell, the recording was likely kept secret in part for “psychological” reasons.
“Sweden was in a trauma at the time” due to the suspected Soviet incursions, he said.
The new analysis of the recording is supported by several facts, including that the Amalia sounds similar to the source in the recording, that the charter boat had a damaged propeller blade, and that the frequency of the vessel’s engine firing rate (EFR) is present in the recording.
While the recording in question is no longer considered proof of a Soviet vessel penetrating Swedish waters, other evidence of Soviet sub incursions remains, including the grounding of a whiskey class submarine on the shores of southern Sweden in 1981.