Russian investigators are considering a terror angle to the plane crash in Kazan which killed all the 50 people on board.
Hours after the Tatarstan Airlines-operated Boeing 737 exploded when trying to touch down, Russian authorities have begun probing the crash.
"All possible causes will be considered - weather conditions, piloting error, technical defects of the aircraft, fuel problems and also terrorism," said Marat Zaripov, first deputy chief of Tatarstan department of the Investigative Committee.
Experts are collecting evidence from the crash scene, but the flight recorder is yet to be found. A criminal inquiry has also been launched while authorities continue to question witnesses.
A team of experts from the Russian Emergencies Ministry in Moscow has flown into Kazan to join the investigation.
Flights from and to Kazan airport have been temporarily suspended.
Samples of the plane's fuel are also being collected to find possible clues. The passenger jet had refuelled in Moscow from where it flew to Kazan.
"Corresponding studies will be ordered after the examination of the crash scene. Documents have been retrieved," Zaripov told reporters.
Rosaviatsiya, the Russian agency which oversees civil aviation, has also set up a special commission to closely study the operations of Tatarstan Airlines.
The authorities are finding it difficult to identify the bodies of the victims as all of them have been burnt beyond recognition. Establishing the identities is expected to take several weeks.
Eyewitnesses at Kazan airport have been reported as saying that there were only ashes and mangled remains of the aircraft.
Zaripov added: "The area covered by fragments of the airliner is quite large - 500 by 500 metres. The investigation continues."
The son of Tatarstan's president and a regional chief of Russian Intelligence Agency FSB were killed in the plane crash. Two foreign nationals, a Briton and a Ukrainian, were among the dead.