* Russian jet was almost vertical when it hit ground
* Pilot error, technical problems seen as likely causes
* Flags at half mast as Russian region mourns (Adds video footage)
By Alessandra Prentice
KAZAN, Russia, Nov 18 (Reuters) - Russian investigators sifted through the charred remains of a Boeing 737-500 airliner on Monday in the search for clues about what caused it to crash in a nosedive and burst into flames, killing all 50 people on board.
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 much personal political prestige.
Residents of Kazan wiped away tears as they lay flowers at the airport where the plane crashed in windy weather. Flags flew at half mast in the city 800 km (500 miles) east of Moscow.
In a video that Russian website LifeNews said was footage of the crash, a plane descends almost vertically into the tarmac and explodes in a huge fireball.
The Tatarstan Airlines flight from Moscow had been trying to abort its landing when it nosedived into the runway, killing all 44 passengers and six crew. "The plane just fell," Transport Minister Maxim Sokolov told reporters.
"The plane was vertical, practically vertical."
Sokolov added the black box flight recorders, which were found on Monday, would need to be analysed for more information.
"The whole town is in mourning," said Elvira Khadiulina, a nursery-school teacher who came to the airport to mourn friends who died in the crash. "These people were only a few minutes from being safe on the ground."
Tatarstan Airlines said it was grounding all its Boeing 737 planes pending an investigation into the crash.
"The main versions are pilot error and technical problems, including equipment failure," Alexander Poltinin, a senior regional investigator, said of the crash.
He said the plane's fuel tank had exploded on impact and it could take weeks for all the dead to be identified in the wreckage, scattered over a wide area.
"There are mostly just fragments of bodies, few corpses," local health ministry official Nail Nigmatullin told the Interfax news agency.
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 ... this isn't surprising."
Russia and the other 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.
There had been no technical problems reported with the leased 23-year-old plane prior to the flight and regular maintenance between flights had been conducted, officials said.
Boeing said in a statement it was prepared to provide technical assistance to the investigators.
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. (Additional reporting by Mikhail Antonov and Nikolai Isayev,; Writing By Timothy Heritage, Alissa de Carbonnel and Gabriela Baczynska,; Editing by Gareth Jones and Robin Pomeroy)