Russian investigators sifted through the charred remains of a Boeing 737 airliner on Monday in the search for clues about what caused it to crash and burst into flames, killing 50 people, officials said on Monday.

Sunday's crash raised new concerns about Russia's poor safety record as it prepares to host the Winter Olympics in the southern city of Sochi in February, an event on which President Vladimir Putin has staked a lot of personal political prestige.

Residents of Kazan wiped away tears as they lay flowers at the airport where the plane crashed in windy weather, with some venting anger over Russia's bad safety record. Flags flew at half mast in the city 800 km (500 miles) east of Moscow.

The Tatarstan airlines flight from Moscow had been trying to abort its landing when it crashed, killing all 44 passengers and six crew in a blaze that left little of the plane.

"The plane just fell," Transport Minister Maxim Sokolov told reporters. "The plane was vertical, practically vertical."

But he cautioned that video recordings of the crash were not of high quality and the black box flight recorders would be needed to provide more information on what happened.

"The main versions are pilot error and technical problems, including equipment failure," Alexander Poltinin, a senior regional investigator, said of the crash.

Poltinin said the fire had been extinguished several hours after the crash, but that it could take weeks for all the dead to be identified in the wreckage, scattered over a wide area.

The plane had lost altitude quickly, he said, and the fuel tank had exploded on impact. The local Health Ministry said only two bodies had been found by noon on Tuesday.

The son of the president of the oil-rich province of Tatarstan and the regional head of the FSB intelligence service were named among those killed. The dead also included two foreigners, a Briton and a Ukrainian.

"It's unimaginable - it's awful, just awful," said a man who gave his name as Dmitry as he left flowers at the airport gate.

"Everyone already knows what state our national aviation is in. So, on the one hand this isn't surprising, it's just a shame that people die," he said.

POOR SAFETY RECORD

Russia and the former Soviet republics combined have one of the world's worst air-traffic safety records, with a total accident rate almost three times the world average in 2011, according to the International Air Transport Association.

In Soviet times, flag carrier Aeroflot had a virtual monopoly of the airline industry, but after the collapse of the Soviet Union, many small private companies emerged.

IATA said last year that global airline safety had improved but accident rates had risen in Russia and the ex-Soviet Commonwealth of Independent States.

The government says it is confident about security and safety matters at the Sochi Olympics and the city's international airport has been upgraded for the Games.

Flight U363 took off from Moscow's Domodedovo airport at 6:25 pm (1425 GMT) and crashed just over an hour later, emergency officials said. The leased plane was 23 years old.

There had been no technical problems reported with the plane prior to the flight and regular maintenance between flights had been conducted, officials said.

Two people who said they had flown on the same plane from Kazan to Moscow earlier on Sunday were quoted by Russian media as saying they felt vibrations as they landed in Moscow.

Kazan is the capital of the largely Muslim region of Tatarstan. There was no suggestion of foul play.

A new runway was built at the airport before the World University Games, held in the city in July. Kazan is one of the venues for the soccer World Cup that Russia is hosting in 2018.

Boeing said in a statement it was prepared to provide technical assistance to the investigators.