The Boeing 737 belonging to Tatarstan Airlines was making its second attempt at a landing in Kazan, 720 kilometers (520 miles) east of Moscow, according to Alexander Poltinin, head of the local branch of Russia's Investigative Committee.
The traffic controller at the Kazan airport who contacted the plane before the crash said the crew told him they weren't ready to land as it was approaching but didn't specify the problem.
Marat Zaripov, deputy head of the local branch of the Investigative Committee, initially told reporters that his team would look into all theories, including a terrorist attack. But the Investigative Committee said in a statement later Monday that it was now considering three possible causes: a technical fault, a pilot error or adverse weather conditions.
Poltinin said it could take weeks to identify the remains.
Investigators have found both of the plane's black boxes — which record the plane's performance and the crew's conversations — but said they were damaged.
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. It was confirmed as authentic to The Associated Press by the emergency press service at Kazan airport and other Russian officials.
Magomed Tolboyev, a highly decorated Russian test pilot, said on Rossiya television that it wasn't immediately clear why the crew was unable to land on their first try in good weather.
U.S. National Transportation Safety Board spokesman Eric Weiss said Monday that a team of eight U.S. aviation safety experts were heading to Russia to assist: three NTSB crash investigators, a U.S. Federal Aviation Administration investigator and four experts from the plane manufacturer.
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," Cox said in an interview. "The airplane hit very hard ... it's in a lot of small pieces."
Cox, a former accident investigator for the Air Line Pilots Association, said it's unlikely the accident was the result of any design flaw in the 737, a short- to medium-haul jet. The plane was first introduced by Boeing in 1968, although there have been major changes in subsequent generations.
"It's a great machine," he said.
Friends of the air crash victims gathered on a central square in Kazan on Monday evening to commemorate the victims.
"I can only express my condolences and wish strength and endurance to the relatives of those who died," said Elmira Kalimullina, one of the friends.
Investigators have started looking through the company'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.