The World War II mystery of what happened to a Finnish passenger plane after it was shot down over the Baltic Sea by Soviet bombers appears to finally be solved more than 80 years later.

The plane was carrying American and French diplomatic couriers in June 1940 when it was downed just days before Moscow annexed the Baltic states. All nine people on board the plane were killed including the two-member Finnish crew and the seven passengers — an American diplomat, two French, two Germans, a Swede and a dual Estonian-Finnish national.

A diving and salvage team in Estonia said this week it located well-preserved parts and debris from the Junkers Ju 52 plane operated by Finnish airline Aero, which is now Finnair. It was found off the tiny island of Keri near Estonia’s capital, Tallinn, at a depth of 70 metres (230 feet).

“Basically, we started from scratch. We took a whole different approach to the search,” said Kaido Peremees, spokesperson for the Estonian diving and underwater survey company Tuukritoode OU, explained the group’s success in finding the plane’s remains.

The downing of the civilian plane, named Kaleva, en route from Tallinn to Helsinki happened on June 14, 1940 — just three months after Finland signed a peace treaty with Moscow following the 1939-40 Winter War.

The news about the fate of the plane met disbelief and anger by authorities in Helsinki who were informed it was shot down by two Soviet DB-3 bombers 10 minutes after taking off from Tallinn’s Ulemiste airport.

“It was unique that a passenger plane was shot down during peacetime on a normal scheduled flight,” said Finnish aviation historian Carl-Fredrik Geust, who has investigated Kaleva’s case since the 1980s.

Finland officially kept silent for years about the details of the aircraft’s destruction, saying publicly only a “mysterious crash” had taken place over the Baltic Sea, because it did not want to provoke Moscow.

Though well documented by books, research and television documentaries, the 84-year-old mystery has intrigued Finns. The case is an essential part of the Nordic country’s complex World War II history and sheds light into its troubled ties with Moscow.

But perhaps more importantly, the downing of the plane happened at a critical time just days before Josef Stalin’s Soviet Union was preparing to annex the three Baltic states, sealing the fate of Estonia, Latvia and Lithuania for the next half-century before they eventually regained independence in 1991.

Retrieval by Soviet submarine

The USSR occupied Estonia on June 17, 1940, and Kaleva’s doomed journey was the last flight out of Tallinn, though the Soviets had already started enforcing a tight transport embargo around the Estonian capital.

American diplomat Henry W Antheil Jr, 27, was on board the plane when it went down. He was on a rushed government mission evacuating sensitive diplomatic pouches from US missions in Tallinn and Riga, Latvia, as it became clear Moscow was preparing to swallow the small Baltic nations.

Kaleva was carrying 227kg (500 pounds) of diplomatic post, including Antheil’s pouches and material from two French diplomatic couriers — identified as Paul Longuet and Frederic Marty.

Estonian fishermen and the lighthouse operator on Keri told Finnish media decades after the downing of the plane that a Soviet submarine surfaced close to Kaleva’s crash site and retrieved floating debris, including document pouches that had been collected by fishermen from the site.

This has led to conspiracy theories regarding the contents of the pouches and Moscow’s decision to shoot down the plane. It still remains unclear why precisely the Soviet Union decided to down a civilian Finnish passenger plane during peacetime.

“Lots of speculation on the plane’s cargo has been heard over the years,” Geust said. “What was the plane transporting? Many suggest Moscow wanted to prevent sensitive material and documents from exiting Estonia.”

But he said it could have simply been “a mistake” by the Soviet bomber pilots.

Various attempts to find Kaleva have been recorded since Estonia regained independence more than three decades ago. However, none of them have been successful.

“The wreckage is in pieces and the seabed is quite challenging with rock formations, valleys and hills. It’s very easy to miss” small parts and debris from the aircraft, Peremees said. “Techniques have, of course, evolved a lot over the time. As always, you can have good technology, but be out of luck.”

New video taken by underwater robots from Peremees’ company showed clear images of the three-engine Junkers’ landing gear, one of the motors and parts of the wings.

Jaakko Schildt, chief operations officer of Finnair, described Kaleva’s downing as “a tragic and profoundly sad event for the young airline”.

“Finding the wreckage of Kaleva in a way brings closure to this, even though it does not bring back the lives of our customers and crew that were lost,” Schildt said. “The interest towards locating Kaleva in the Baltic Sea speaks of the importance this tragic event has in the aviation history of our region.”