WARSAW, Poland – Polish prosecutors said Wednesday they are investigating whether Poland's World War II prime minister and chief army commander, Gen. Wladyslaw Sikorski, died accidentally in a 1943 plane crash or was assassinated.

Prosecutors of the state-run National Remembrance Institute said they plan to file a formal request asking to look into classified British WWII files as they probe the crash of the British bomber in British-ruled Gibraltar in which Sikorski, Poland's top politician of the time, was killed.

Poland's Prime Minister Donald Tusk and President Lech Kaczynski have spoken in favor of investigating Sikorski's death.

There is an “existing and justified suspicion of a criminal cause of the death,” which has not been investigated in Poland or elsewhere, the institute said in a statement released Wednesday.

Though it did not name suspects, the institute said it was investigating a “communist crime,” suggesting its suspicions fall primarily on the Soviet Union, which attacked Poland after forming an alliance with Nazi Germany at the start of the war.

Ewa Koj, a leading prosecutor in the institute's Katowice branch, which opened the probe, said on TVN24 television that it will be based on documents and new testimony from people linked to the initial investigations shortly after the crash.

Sikorski's Liberator II aircraft crashed into the sea just 16 seconds after taking off from Gibraltar on July 4, 1943.

The crash came only three months after Joseph Stalin broke diplomatic ties with the Polish government in exile, following Sikorski's demand the International Red Cross investigate the Katyn massacre. The demand came after German forces discovered in the village of Katyn in western Russia and elsewhere the graves of some 21,700 Polish military officers, intellectuals and priests – executions later proven to have been carried out by the Soviets.

Investigators may also need to open Sikorski's tomb at the Renaissance Cathedral of the Wawel Castle in Krakow, where his remains rest among those of many Polish kings and top figures.

Sikorski, who was chief commander of Poland's armed forces, made a stopover at Gibraltar as he was returning from an inspection of forces in the Middle East. He was also the prime minister of Poland's government-in-exile based in London, strongly pushing Polish interests with the Western Allies and the Soviet Union.

A British probe into the crash blamed it on jammed controls, but a separate Polish investigation did not rule out sabotage. Polish historian Dariusz Baliszewski has also said his research suggests Sikorski could have been assassinated.

For decades, Sikorski's death has generated theories that he was murdered, alternatively by British, Soviet or even Polish factions. One theory holds that British double agent Kim Philby – who was on Gibraltar at the time of the crash and who defected to the Soviet Union in the early 1960s – might have had a role in the crash.

Sikorski's death “influenced Poland's wartime fate,” Baliszewski said on TVN24. “For the sake of truth we should know what really happened.”