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Poland's new government relaunched an inquiry on 4th February 2016 into the death of President Lech Kaczynski in a plane crash in Russia in 2010, a move likely to strain Warsaw's relations with its former overlord, already fragile over the Ukraine crisis.
An inquiry by the previous government returned a verdict of pilot error but the winner of Poland's October election, the Law and Justice (PiS) party led by Kaczynski's twin brother Jaroslaw, says an onboard explosion could have caused the crash.
Speaking at a ceremony to announce the decision, Defence Minister Antoni Macierewicz also hinted at an explosion as a possible cause, saying the plane had "disintegrated" metres above the ground before crashing.
"There is no doubt that these circumstances are not only a sufficient reason, but one that makes it compulsory to reexamine this tragedy," Macierewicz said.
Though the PiS has never accused Russia of orchestrating the president's death it has said the Kremlin benefited from the crash, which also killed the central bank chief, top army brass and several lawmakers, triggering a period of political turmoil.
PiS officials have also accused Moscow of prolonging its own investigation and withholding evidence, including the black box flight recorders and wreckage from the plane.
Russia has kept the wreckage for nearly six years and says it cannot returned until its own criminal probe is concluded.
Separately, the Polish prosecutors said on Thursday Russia had refused to help them bring charges against two Russian air traffic controllers involved in guiding the presidential aircraft.
Poland has been one of the most outspoken critics of Russian policy towards a pro-Russian separatist rebellion in eastern Ukraine, joining Western allies in accusing Moscow of supplying help to the insurrection - something the Kremlin denies.
The crash took place near Smolensk, western Russia, close to the place where Stalinist secret police forces shot some of the 22,000 Polish officers and intellectuals they executed in 1940. For decades, Moscow blamed Nazi Germany for the mass executions.
The massacre is an enduring symbol for Poland of its suffering at Soviet hands, and president Lech Kaczynski had been flying in to commemorate it.
While the crash initially united Poles in grief, it has since given rise to bitter domestic political divisions.
The late president's twin brother has repeatedly accused then prime minister Donald Tusk, now head of the European Council, of being indirectly responsible for the crash through negligence.
The previous state probe produced no evidence of that. But earlier this year, a Polish court refused to dismiss outright a case against Tusk's former chief of staff and two of his aides, brought to court by some of the victims' relatives. (Additional reporting by Anna Koper in Warsaw, Denis Dyomkin and Lidia Kelly in Moscow; Editing by Jon Boyle and Dominic Evans)