Two Russian airliners carrying 89 passengers and crew have crashed within minutes of each other after flying out of the same Moscow airport.
Investigators are searching the wreckage for clues to the crashes
Witnesses at one scene said they heard a plane roaring overhead followed by "an explosion like thunder".
Russian President Vladimir Putin ordered the state security services, the FSB, to launch an investigation.
An FSB spokesman said searches had revealed no signs of terrorism but sabotage had not been ruled out.
There was speculation Chechen rebels were behind the attack, ahead of presidential elections in the republic on Sunday - but this has been denied by rebel leaders.
A spokesman from Moscow's Domodedovo airport said no foreigners were on either passenger list.
The two planes flew out of the airport within 40 minutes of each other on Tuesday evening.
Trouble was noticed with both flights almost simultaneously at about 2300 local time (1900 GMT).
Wreckage from the first plane, a Tu-134 bound for Volgograd, was found near the village of Buchalki, in the Tula region, about 200km (125 miles) from Moscow.
"First there was the sound of roaring, as if the plane was flying very low, then came an explosion, like thunder, followed by two more blasts after a couple of seconds. And that was it," said eyewitness Yevgeny Chorkin.
An official later said all 43 passengers, including nine crew, had been killed.
The small Volga-Aviaexpress airline, which owned the plane, said all necessary security checks had been completed, and it was being piloted by the firm's director, Yury Baichkin.
The second plane, a Tu-154 heading for the Black Sea resort of Sochi with 38 passengers and eight crew on board, disappeared from the radar at around the same time.
Bad weather hampered the search for wreckage, which was found about nine hours after the plane's disappearance near the southern Russian town of Rostov-on-Don, about 1,000km (600 miles) south of Moscow.
A spokesman for the plane's owners, Russian air company Sibir, said they received an "automatically generated telegram from the Sochi air control centre that the plane had been hijacked".
But Russia's Interfax news agency later quoted an unnamed law enforcement source as saying it had been a distress alert.
President Putin, on holiday in Sochi, ordered the FSB to investigate the crashes.
FSB investigations are normally held only when an accident occurs in suspicious circumstances, a security source told Reuters news agency.
"According to a preliminary conclusion made by the investigation teams which are working at the sites of the crashes, no signs of terrorist acts or explosions have been found," FSB spokesman Sergey Ignatchenko told Itar-Tass news agency.
Relatives faced an anxious wait for news
"Several theories are being considered, the main one is illegal interference into the work of civil aviation."
Security has been tightened at Russian airports.
When told of the two crashes, Russia's UN ambassador, Andrey Denisov, said: "Now we have to see if there's terrorism," reported the Associated Press news agency.
One aviation source quoted by Interfax said: "The fact that both planes took off from one airport and disappeared from radars around the same time can show it was a planned action."
There were fears militants linked to a bloody uprising in the southern republic of Chechnya may be behind the crashes.
Correspondents say Chechen rebels had threatened to disrupt elections in Chechnya to replace President Akhmad Kadyrov, who was killed by a rebel bomb in May.
But Akhmed Zakayev, spokesman for the Chechen separatist leader Aslan Maskhadov, ruled out any involvement.
"Maskhadov is not linked to this in any way," he reportedly told Moscow Echo radio.