Poland exhumed the corpse of its late president Lech Kaczynski on Monday, six years after he died in a plane crash in Russia that has spawned a litany of conspiracy theories and riven Polish society.

Investigators have opened the sarcophagus of Kaczynski and his wife as part of a new inquiry into the April 2010 crash in Smolensk that killed all 96 people on board, amid allegations by Poland’s government that their deaths were no accident.

Head of Poland’s ruling Law and Justice party Jaroslaw Kaczynski, twin brother of the late president, has long claimed that foul play was to blame for the crash, Poland’s worst national tragedy since the second world war. Claims that the Russian state or his liberal opposition were complicit has become an integral part of the party’s political message to nationalist voters.

"Poland will not be truly free without the truth … without closing the Smolensk case," Mr Kaczynski said last week. "This has cast a long shadow on the life of our nation, our society."

After waiting until the last public visitors had left the crypt on Krakow’s Wawel Hill where Kaczynski and his wife lie in state, investigators on Monday evening opened the coffins containing the former first couple ahead of four days of tests on their corpses. Proponents of the exhumations, which will involve almost all those killed in the crash, say the initial postmortem examinations provided insufficient detail.

The crash, which took place in heavy fog on the forested approach to the runway in Smolensk, is a lightning rod for political conspiracy theories in Poland, where sensitivity over foreign — and particularly Russian — influence on public life is high.

Russia has so far refused to return the plane wreckage to Poland, stoking claims that the tragedy was an orchestrated attack. That the passengers, who included Poland’s central bank and National Security Bureau heads and its military chiefs of staff, were on the way to attend a memorial in Katyn where more than 21,000 Poles were murdered by Soviet soldiers during the second world war has only added to the conspiracy theories.

Official investigations by the Russian and Polish authorities both found that pilot error had caused the crash, while the Polish probe also suggested mistakes were made by Russian air traffic controllers.

Recent polls estimate just over a quarter of Poles believe the crash was not an accident. Yet soon after it came to power in November, Law and Justice announced a fresh investigation involving international experts and led by defence minister Antoni Macierewicz, who has long claimed that the plane disintegrated in mid-air before it hit the ground, possibly after an on-board explosion.

Mr Kaczynski has alleged that his longstanding political adversary Donald Tusk, Poland’s prime minister at the time of the crash and current president of the European Council, bears responsibility for the crash or a subsequent whitewash.