Russian investigators are searching for clues into what caused two airliners to crash within minutes of each other after leaving the same Moscow airport.
The planes were found hundreds of miles apart
Officials say they have not ruled out terrorism, although they have not yet found evidence of sabotage.
Eighty-nine passengers and crew died when the aircraft went down about 800km (500 miles) apart.
One of the planes sent a distress signal, which may have indicated a hijacking, before contact was lost.
There is speculation that Chechen rebels were involved, ahead of presidential elections in the republic on Sunday - but this has been denied by rebel leaders.
Security stepped up
Russian President Vladimir Putin ordered the FSB security service to find out what caused the crashes.
Security has reportedly been stepped up at Moscow's airports and other public sites.
Russia's chief prosecutor Vladimir Ustinov told Mr Putin investigators were considering "a number of versions, among them a terrorist act, and human and technical factors", Reuters news agency reported.
FSB spokesman Sergei Ignatchenko said there was yet no evidence of terrorism.
"The main version is violation of planes' maintenance. Judging by the fragments of the planes, it is possible to say that there were no terrorist acts on the planes."
But the operator of one of the planes, a Tu-154, said there were indications there had been an explosion onboard.
"The wide distribution of large fragments indirectly confirms the conjecture that the plane broke up in midair because of an explosion," Sibir Airlines said in a statement.
Earlier, Sibir said it had received an "automatically generated telegram from the Sochi air control centre that the plane had been hijacked".
Interfax and Itar-Tass news agencies later quoted an unnamed law enforcement source as saying it had been a distress alert.
Both aircrafts' "black box" flight data recorders have been recovered and sent to Moscow to be decoded.
The BBC's Sarah Rainsford in Moscow says the near simultaneous loss of two flights is certainly being treated as suspicious.
She says aviation experts there say the chances of this being pure accident are one in a million.
Noise 'like thunder'
The two planes flew out of Domodedovo airport within 40 minutes of each other on Tuesday.
Contact with both aircraft was lost 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 witness Yevgeny Chorkin.
Relatives faced an anxious wait for news
Officials said all 43 passengers and crew had been killed.
A spokesman from Domodedovo airport said no foreigners were on either passenger list.
But Itar-Tass news agency quoted an Israeli embassy official as saying two Israeli citizens from St Petersburg were on the Tu-134, owned by Volga-Aviaexpress airline.
The second plane, the Tu-154, was heading for the Black Sea resort of Sochi with 46 passengers and crew.
Wreckage was found near the southern Russian town of Rostov-on-Don, some 1,000km (600 miles) south of Moscow.
There were fears that 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, said Mr Maskhadov was "not linked to this in any way", Moscow Echo radio reported.