Russia is observing a day of mourning for the 89 victims of a twin air crash, the cause of which remains a mystery.
In both cases debris was scattered over a wide area
Experts have recovered the flight data recorders from the two Tupolev airliners, which crashed within minutes of each other over southern Russia.
"There is still no clear-cut idea about what happened," said Transport Minister Igor Levitin on Thursday.
Officials say they have not ruled out terrorism, although they have not yet found any evidence of sabotage.
Russian flags flew at half-mast on Thursday and light entertainment was withdrawn from theatre and television schedules.
Eighty-nine passengers and crew died when the aircraft - a Tu-134 and a Tu-154 - crashed about 800km (500 miles) apart on Tuesday night after leaving Moscow's Domodedovo airport.
One of the planes sent out 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.
Russian President Vladimir Putin ordered the FSB security service to find out what caused the crashes and a government commission has started an investigation.
Security has reportedly been stepped up at Moscow's airports and other public sites.
Mr Putin has also ordered the government to draft legislation to hand over responsibility for airport security to the interior ministry.
He interrupted his holiday in the Black Sea resort of Sochi to return to Moscow after news emerged of the crashes.
There is speculation that poor quality fuel or human error might have been to blame.
The victims were all Russian nationals, apart from one Israeli, Itar-Tass news agency reported.
The families are to receive 112,000 roubles ($3,800) each in compensation - unless it is proven that terrorism was to blame, in which case they would receive less.
FSB spokesman Sergei Ignatchenko said there was 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 of an explosion on board.
"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.
Sibir said it had received a distress alert from its jet shortly before the Tu-154 disappeared from radar screens.
Relatives are going to the crash sites to help identify the dead
The BBC's Sarah Rainsford in Moscow says the near simultaneous loss of two flights is certainly being treated as suspicious.
The two planes left 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. The jet was owned by the Volga-Aviaexpress airline.
Officials said all 43 passengers and crew had been killed.
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.