A woman mourns near flowers left outside Kazan airport, where a Tatarstan Airlines Boeing 737 crashed, Nov 18, 2013. [Photo/Agencies]

MOSCOW - The Russian republic of Tatarstan declared a day of mourning Monday for all 50 victims of an air crash that occurred Sunday night in the capital of Kazan.

The Tatarstan Airlines Boeing 737, bound from Moscow to Kazan carrying 44 passengers and six crew members aboard, crashed at about 7:30 pm (1530 GMT) during its second attempt to land in the airport of Kazan, a city located some 800 km east of Moscow.

Those killed included Irek Minnikhanov, a son of Tatarstan President Rustam Minnikhanov, and Alexander Antonov, head of the regional Federal Security Service.

A Briton and an Ukrainian were also among the victims, local emergency authorities said. The rest of the victims are all believed to be Russians.

The Chinese Embassy in Russia confirmed to Xinhua Monday that no Chinese were among the crash victims.

Deputy Emergency Situations Minister Vladimir Stepanov said not all the bodies had been located and the process of retrieving would be completed Monday, adding that wreckage cleanup could last for a few more days.

On Monday morning, the Emergency Situations Ministry sent a group of additional rescuers and psychologists to the crash scene as more than 1,200 people and 240 vehicles were working at the site.

The flight recorders had not been found so far. Investigators said possible causes of the tragedy included technical malfunction and pilot error.

Marat Zaripov, first deputy head of the Tatarstan Branch of the Investigative Committee, the country's criminal investigative body, said all possible causes for the crash would be considered, including terror attack.

"All possible causes will be considered: weather conditions, piloting error, technical defects of the aircraft, fuel problems and also terrorism," Interfax news agency quoted him as saying.

The plane owned by Tatarstan Airlines, the regional carrier of the Tatarstan region in central Russia, had been in service for 23 years and had seven different owners during its life, Russian media reported.

The plane's fuel tank exploded as the airliner was about to fly a second circle before landing, confusing some experts as the fuel should be exhausted after the one-hour flight.

According to Zaripov, the airliner was refueled in Moscow before taking off. Instructions were given to take samples of the fuel.

Russian President Vladimir Putin expressed condolences to the families of the victims and instructed a committee to probe into the fatal accident.

Family members of the killed passengers each would receive compensation of about 2 million rubles (some $61,530).