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Polish investigators reviewing what caused a 2010 airplane crash in Russia, that killed then-president Lech Kaczynski and 95 others, have alleged that the black boxes were tampered with in the original probe.
The new investigation was launched by the governing right-wing Law and Justice (PiS) party, which swept back to power in October after eight years in opposition and whose leader is the twin brother of the late president.
PiS politicians have long insisted that the presidential jet crash was no accident, even though both Polish and Russian investigators found that pilot error, bad weather and poor air-traffic control were to blame.
Most of the others who died when the plane came down in Smolensk, western Russia, were senior Polish state officials, including the military chief of staff and central banker.
"Some of the elements were tampered with," commission of enquiry head Waclaw Berczynski told reporters on Thursday.
His colleague Kazimierz Nowaczyk alleged that three seconds had been cut from one of the black-box recordings, while five seconds were deleted from the other.
The commission members outlined information they claimed showed there had been bias in the original Polish investigation led by then-interior minister Jerzy Miller.
They played a recording at the news conference on which they said Miller could be heard instructing someone to match the Polish report to the Russian one so that the public would not think "we are hiding something" and to "prevent myths from being formed".
Also at the news conference was Frank Taylor, a British aviation expert who took part in the investigation of the 1988 Pan Am flight that was brought down by a bomb over the Scottish town of Lockerbie.
Pointing to weaknesses in the original Polish probe, he alleged that satellite photos showed that wreckage had been moved after the crash.
A 2015 Polish opinion poll found that 22 per cent of respondents believed the crash was an assassination.
The new investigation's findings were contested by an expert who worked on the original report.
Maciej Lasek, head of the Committee for Investigation of National Aviation Accidents, speaking on TVN24 commercial television, said he would have liked to have seen the work of the new commission in advance and know its methodologies.
"These are just words," said Lasek, who participated in the first enquiry.
"None of the new investigators visited the accident site" in Smolensk, he said, adding that the accusations of rigging results is due to "an absolute lack of understanding of the principles of the black boxes and the synchronisation of data".
