Investigators have identified a second non-crew voice captured by cockpit recordings extracted from the crashed Polish presidential Tupolev Tu-154M.
The Russian Interstate Aviation Committee (MAK) has not given a name for the individual, but says it has "identified the second specific person" whose phrases were listed in the transcript of the cockpit conversation.
One other non-crew voice has already been confirmed as belonging to Polish air force commander Andrzej Blasik.
While a version of the transcript from the MARS-BM cockpit recorder was formally handed to Polish authorities on 31 May, several sections of the conversation were rendered unintelligible by noise.
MAK says it is working specifically to clarify two phrases from the previously unidentified non-crew voice.
The Tu-154 crashed on approach to Smolensk in dense fog on 10 April, after sinking below the correct approach path, but no explanation has yet been given for the decision to attempt the landing in conditions far below minima.
Specialists in the audio spectrum analysis are attempting to refine the transcript of the cockpit discussion, to obtain a clear understanding of the reasoning that led to the fatal approach.
Russian authorities have also recruited test pilots from the Gromov research institute as well as the military, along with experienced Tu-154 cockpit personnel and aviation psychologists, to analyse the Polish flightcrew's actions.
Analysis of the terrain-awareness warning system and the flight-management system on board the ill-fated Tu-154 shows both were providing information to the crew correctly. This research was carried out in the facilities of Universal Avionics Systems in the USA.