The death of the Polish World War II leader, General Wladyslaw Sikorski, in an air crash is one of the great unsolved mysteries in Polish history. Investigators are exhuming his body to clarify allegations that he may have been murdered.

Sikorski died in July 1943 in a British Royal Air Force plane, which crashed into the sea off Gibraltar just seconds after take-off, killing all on board except the Czech pilot.

An official British investigation into the crash concluded it was an accident caused by a technical malfunction. But not everyone is convinced, and some historians believe he was murdered.

Now Polish prosecutors want to find out for sure by exhuming his body for forensic investigation.

Polish researcher Marek Lasota from the Institute of National Remembrance said he and his colleagues “were considering it a communist crime but we don’t blame the Russians”.

He said there was “no hostility”, that they “simply want to verify all doubts about the case and our investigators will hopefully be able to clear up the circumstances surrounding the death,” Lasota said.

Investigators have begun digging up his body from its resting place at Krakow Cathedral and say it’ll take a few weeks before their forensic research is complete, which may unravel the secrets of one of Poland's most intriguing historical mysteries.

But why has it taken 65 years for such a probe to take place?

The Soviet Union barred any investigation in Poland, and historians have only recently been given access to previously secret files. Polish prosecutors say these have given them enough new evidence to investigate if their country's wartime leader was murdered. They believe that the Soviets were to blame.

The crash came shortly after Soviet leader Joseph Stalin broke diplomatic ties with the wartime Polish government-in-exile, following Sikorski’s demands for an investigation into the Katyn Forest massacre. This was the murder of thousands of Polish citizens and officers, later proven to have been carried out by Soviet forces.

Conspiracy theorists argue that Stalin wanted Sikorski dead because he was too strong a leader and a hindrance to the Soviet leader's relations with his new western allies.

This ties in with another theory that the notorious British-Soviet double agent Kim Philby – who was stationed with British intelligence in Gibraltar at the time – could have had a hand in the crash.

But it’s not a hypothesis supported in Moscow, as historian Albina Noskova says:

“Moscow didn’t want a breaking-off of relations. There are even reports that two days before Sikorski’s death he planned to visit Moscow to try to resume negotiations, even at the cost of the eastern border between the Soviet Union and Poland.”

So, it’s a case full of question marks and intrigue – compounded by the fact that many British documents relating to the crash remain classified and under wraps. Their “top secret” status in London has sparked some suspicion that the British themselves could have orchestrated the murder.

But historian and Polish Member of the European Parliament, Professor Wojciech Roszkowski, doesn’t believe it’s “likely” that Sikorski was murdered.

“He was in the way and an obstacle – mostly for the Soviets but also for the Americans and the Brits – but I don’t think they would really kill him.”

There are numerous theories about the death of General Sikorski, but prosecutors in Poland are hoping they’ll settle the mystery and put an end to years of speculation.