Russian investigators have discovered that the information cockpit-voice recorder from the crashed Katekavia Antonov An-24 contains incomplete information about the accident.
The Interstate Aviation Committee (MAK) says the recording only features the crew conversations for the first 80min of the fatal flight.
MAK says, however, that the flight-data recorder has data from the last 90min.
All 11 passengers, plus the flight attendant, were killed after the aircraft came down on approach to Igarka, in northern Russia, on 2 August. Only the flight crew - two pilots and an engineer - survived.
Federal air transport regulator Rosaviatsia says the aircraft, which had been arriving from Krasnoyarsk Cheremshanka Airport, had been attempting to land at night, in rain, with visibility of 3,500m.
It says the approach was being conducted using outer and inner NDB navigational aids.
But it states that, as the An-24 prepared to land, it "deviated to the right of the approach path", struck the ground, broke up and caught fire.
Rosaviatsia says the aircraft's commander was highly experienced, with 15,800hr on An-24s, while the co-pilot had 2,860hr on type.
It adds that the 36-year old aircraft had last undergone scheduled maintenance in May this year, although it had a line check on the day of the accident.