The commission said at a press conference on Thursday, September 15, that black-box recordings were "manipulated and shortened", adding that a copy of the recordings obtained from Russia by Poland was missing seconds of "crucial data," Radio Poland reported.
Neither the wreckage of the plane, which crashed in western Russia, nor its black boxes, have been handed over to Poland.
Kazimierz Nowaczyk, deputy head of the commission, said that "the committee found the source material […] of the Polish and Russian black boxes.
"Three seconds were cut out from the Polish recordings, and five seconds from the Russian one."
Read alsoNew recording emerges in Smolensk crash caseNowaczyk added that the present commission was able to "decipher" five additional seconds of recordings.
"We have already found clues which show that the first engine had failed, the generator was malfunctioning, and the two radio altimeters were not in working order," Nowaczyk said.
The commission on Thursday also played back a recording reportedly from a sitting of the previous government's commission charged with probing the crash.
The current commission said that in the recording, former Interior Minister Jerzy Miller can be heard saying that the official "report must be consistent and cannot raise any doubts."
The commission indicated that Miller had tried to convince the previous commission, which was set up days after the crash in 2010, to stick closely to the Russian version of events in their official report.
The contents of these recordings were reported by a newspaper earlier this week.
A significant part of the conference was focused on a birch tree on the flight path during the final approach in heavy fog at the Smolensk military airport in the early hours of April 10.
The previous Miller commission had suggested that the tree could have clipped the presidential plane's wing, causing the crash.
However, a presentation during the September 15 conference showed that parts of the debris of the plane wreckage were found some 50 meters away from the tree, in the opposite direction to the plane's flight path – away from the airport.
"This seemed to suggest that it is unlikely that the tree could have been a contributing factor to the crash," Marcin Wiklo, from the wSieci weekly, told the TVP broadcaster after the conference.
The commission on Wednesday, September 14, met the families of the victims of the crash, who were presented with "significant new" information related to the disaster.
Some commentators said that they had expected more revealing information in Thursday's press conference, attended by Defense Minister Antoni Macierewicz.
"The commission has only just started their investigation, and I'm sure that their body of work will increase over time," wSieci's Wiklo said.
Read alsoPoland to exhume remains of all victims of Smolensk crash – mediaEarlier this week, the commission's head, Waclaw Berczynski, said that one of the discrepancies discovered by his team was related to the distance to the edge of the runway reported to the Polish crew by the Russian control tower.
"The difference is substantial and it could have affected the landing operation," Berczynski told the TVP broadcaster.
"The Tupolev crew was misinformed. We don't know if it was intentional, though," Berczynski said.
The Polish president's Tupolev plane crashed in Smolensk, western Russia, on April 10, 2010, killing all 96 people on board, including the head of state, Lech Kaczynski.