An airport security camera video has captured a doomed Russian airliner plunging to the ground in a near-vertical crash, killing all 50 on board.
The crash at Kazan airport was shown by Russian television stations, raising a host of questions, including why the plane's second attempt to land at night in good weather went so horribly wrong.
US aviation experts were heading to the scene to help Russian investigators, who were combing through the incinerated wreckage of the Boeing 737.
The plane, belonging to Tatarstan Airlines, crashed on Sunday night at its home port of Kazan, 500 miles east of Moscow.
It was making its second attempt at a landing, according to the local branch of Russia's Investigative Committee, which said investigators were looking into possible pilot error or equipment failure.
The air traffic controller who contacted the plane before the crash said the crew told him they were not ready to land as it was approaching but did not specify the problem.
The brief video taken by an airport security camera showed the plane going down at high speed at a nearly vertical angle and then hitting the ground and exploding.
Investigators have found both of the plane's black boxes but said they were damaged. The boxes - which record the plane's performance and the crew's conversations - are essential for the crash probe.
John Cox, an aviation safety consultant who flew 737s for 15 years for US Airways, said one of the first issues investigators will look at based on the nearly vertical angle of descent in the video will be whether the plane experienced an aerodynamic stall, which usually occurs when a plane slows to the point where its wings lose lift.
"Anytime you have an airplane that gets this vertical, the immediate suspicion is that it stalled," he said.
Investigators have started checking the airline's records, which showed the plane was built 23 years ago and had been used by seven other carriers prior to being picked up by Tatarstan Airlines in 2008.
In 2001, it was damaged in a landing accident in Brazil that injured no one and t he company insisted that the aircraft was in good condition for the flight.
The carrier has had a good safety record but appears to have run into financial problems recently. Its staff went on strike in September over back wages, and the Kazan airport authority has gone to arbitration to claim what it said was Tatarstan Airlines' debt for servicing its planes.
Industry experts have blamed some recent Russian plane crashes on a cost-cutting mentality at some carriers that neglects safety in the chase for profits. Insufficient pilot training and lax government controls over the industry also have been cited as factors affecting Russian flight safety.
Former Finance Minister Alexei Kudrin said that the government should tighten its oversight of carriers and subsidise the upgrading of their fleets to improve safety.
Russia's last deadly airliner crash was in December, when a Russian-made Tupolev belonging to Red Wings airline careered off the runway at Moscow's Vnukovo airport. It rolled across a snowy field and slammed into a slope, killing five of its eight crew members.
A 2011 crash in Yaroslavl killed 44 people including a professional hockey team and was blamed on pilot error.