Preliminary investigation into the fatal Angara Airlines Antonov An-24 ditching in the River Ob has determined that the crew initially intended to divert to Nizhnevartovsk airport.
The Russian Interstate Aviation Committee (MAK) said the pilots had received an oil contamination alert before the left-hand engine caught fire.
Operating between Tomsk and Surgut on 11 July the turboprop aircraft had been cruising at 6,000m (19,700ft) when signs of a problem developed.
MAK has obtained initial information from the cockpit-voice and flight-data recorders following the crash.
The cockpit instrument informed the crew that there were particles in the oil in the left-hand engine, said MAK.
"The crew began a descent and, in agreement with Nizhnevartovsk air traffic control, decided to land at Nizhnevartovsk airport," said MAK.
"During the descent procedure a fire broke out in the left engine. The fire was not eradicated by application of two extinguisher discharges."
Seven occupants of the An-24 were killed when the crew decided instead to attempt a landing on the river.