TALLINN, June 4, 2008 (AFP) - A US military probe has failed to find the wreckage of a passenger plane thought to have been shot down by the Soviet Union during WWII, claiming the life of an American diplomat, a official said Wednesday.

Eric Johnson, spokesman for the US embassy in Estonia, told AFP that the six-day operation in the Baltic Sea by a US Navy oceanographic survey ship had been called off on Wednesday morning.

Estonia's defence ministry had asked the United States to help solve the mystery over the fate of the 'Kaleva', a Finnish passenger aircraft which crashed into the Baltic shortly after taking off from Tallinn on June 14, 1940.

The aircraft was carrying nine passengers and crew, all of whom died.

Among the victims was Henry Antheil, a courier, whose name was last year added to a US State Department memorial to American diplomats killed in the line of duty.

At the time, Estonian fishermen said they had seen the Helsinki-bound aircraft being attacked by the Soviet air force.

The incident took place as the Soviet Union was bracing to invade Estonia.

In the summer of 1939, Moscow had cut a deal with Nazi Germany giving it a green light to take over Estonia and the other Baltic states of Lithuanian and Latvia, as well as grab a slice of nearby Poland.

The Soviets gradually ratcheted up the pressure on the Baltic states over coming months, forcing them to agree to the eventual deployment of Red Army troops.

In 1940 Moscow went a step further.

It imposed an air and sea blockade on Estonia on June 14, and then launched a full-scale invasion two days later.

It also took over Lithuania and Latvia.

International attention was meanwhile focused on western Europe, where Nazi Germany had occupied Paris the same week.

In addition to the fact that 68 years have gone by since the incident, there may be another reason for the failure to locate the wreckage of the aircraft.

There are long-standing rumours that the Soviets later raised the plane in secret.

Moscow had ample opportunity to do so, because it ruled the Baltic states until the collapse of the communist bloc in 1991.

'We can't comment on speculation over whether the wreckage was lifted by the Soviets. We do not have such information,' Johnson said.

'Studies to find the wreck are not going to continue in the nearest future. At least not this summer,' he added.