Investigators were also looking at a witness video of the abortive flight and the plane's plunge into the sea. The Tu-154 jet, whose passengers included more than 60 members of the internationally renowned Red Army Choir, was heading to Russia's military airbase in Syria on Sunday when it went down off the coast of Sochi shortly after take-off from a refuelling stop at the airport.
Investigators said they have sorted and documented thousands of passengers' personal items and identification documents, questioned locals and are checking the fuel equipment at the airport. One witness "filmed the takeoff, flight and fall of the plane into the sea," the Investigative Committee said in a statement. The discovery of the black box comes as searchers scramble to recover bodies and remaining debris from the aircraft in an operation involving divers, deepwater machines, helicopters and drones. The defence ministry said that five plane fragments, including part of the fuselage and engine, were found overnight 30 metres (98 feet) underwater at around 1,700 metres from the shore. Searchers later found an additional three fragments, including landing gear and a portion of the engine, the ministry said.
Russia's federal security service has said it is looking into four suspected causes of the crash, namely pilot error, technical failure, faulty fuel and an object in the engine. The defence ministry said a total of 12 bodies and 156 body fragments had been recovered from the sea since the crash, all of which are being sent to Moscow for DNA identification. The Kommersant daily newspaper reported that investigators are relying on a witness statement by a coastguard member who saw the plane in its final moments descending towards the sea with its nose tilted sharply upward. Authorities have not said how long it would take to decipher the black box.