WARSAW, September 3, 2008 (AFP) - Polish officials on Wednesday opened a probe into the death of Poland's World War II prime minister in exile General Wladyslaw Sikorski in a 1943 plane crash in Gibraltar that some believe was the work of the Soviet Union.

According to Poland's Institute of National Remembrance (IPN), responsible for probing Nazi and communist-era crimes, the crash bears the hallmarks 'of a criminal act'.

Sikorski, often compared to France's wartime General Charles De Gaulle, died on July 4, 1943 when the British Royal Air Force aircraft carrying him plunged into the sea seconds after take-off from Gibraltar.

The Czech pilot, the sole survivor among the 17 people aboard, testified that the plane's controls had jammed.

The Britain-bound Liberator aircraft had already flown without a hitch from the Middle East, where Sikorski had been inspecting Polish troops.

At an RAF inquiry days later, a forensics officer said the victims' bodies 'showed head injuries and multiple injuries' suggesting they had died in the crash.

The inquiry ruled that the plane became 'uncontrollable for reasons which cannot be established,' public records show.

That, plus two previous unexplained incidents involving planes carrying Sikorski, the presence of a Soviet diplomatic aircraft in Gibraltar on July 4, and the fact that many British documents about Sikorski's death remain classified, have helped conspiracy theories flourish for decades.

Historians who believe Sikorski was assassinated stress that in 1943 the Polish government-in-exile in London became an uncomfortable ally to both Western states and the Soviet Union.

Sikorski refused to remain silent about Soviet dictator Joseph Stalin's responsibility for the 1940 massacre of some 22,000 Polish officers in and near western Russia's Katyn forest.

While Stalin falsely accused Nazi Germany of being responsible for the massacre, following the carve-up of Poland in the 1939 Nazi-Soviet pact, Sikorski demanded a probe to discover who was truly responsible.

As the Soviet Union crumbled in the early 1990's Russian leaders admitted Moscow had committed the Katyn massacre but did not classify it as a war crime or act of genocide.

In an effort to verify the circumstances of Sikorski's death, researchers also want to exhume his remains from the Wawel Castle crypt in the southern city of Krakow.

Scientists from Krakow's Jagiellonian University say even a tardy autopsy could prove whether Sikorski died from injuries sustained in the crash or from gunshot wounds. Exhuming Sikorski's remains requires his family's approval.