Non-crew members were present in the cockpit of the plane of Polish president Lech Kacyznski that crashed in Russia last month.
All 96 passengers died when the plane went down in Smolensk, western Russia.
One of the non-crew members has been identified, though he or she was not named by officials.
“It has been established that in the cockpit there were individuals who were not members of the crew,” said Tayiana Anodina, the head of the crash investigation.
“The voice of one of them has been identified exactly, the voice of the other, or the others, will require additional information from the Polish side,” she added.
Investigators have already ruled out terrorism as one of the possible causes of the accident.
The crash has been described as Poland’s worst postwar tragedy. It also claimed the lives of the Polish first lady and several senior and military officials.
The presidential delegation had been heading to a memorial service for the victims of Katyn massacre when the plane crashed.