Two Russian passenger planes crashed almost simultaneously last night, killing all 89 people on board in what investigators believe might have been a terrorist attack.
The planes, which belonged to two different companies and were bound for different destinations, took off from Moscow's Domodedovo airport around an hour apart last night and crashed within minutes of each other.
President Vladimir Putin ordered the FSB security service to investigate the case, something it is normally only asked to do where terrorism is suspected.
Security has been tightened at all Russian airports since the crashes.
Later in the day Mr Putin broke his summer holiday in the Black Sea resort of Sochi and returned to Moscow, the Kremlin said.
Fear of attacks in Russia is already high ahead of next Sunday's presidential election in Chechnya which separatist rebels have vowed to disrupt.
A Tu-134 flying to Volgograd went down near the town of Tula south of Moscow. Within minutes and 800 km away, a Tu-154 bound for Sochi crashed near the southern town of Rostov-on-Don.
The owner of the Tu-154, Sibir Airlines, said the pilots had triggered a hijack alert just before their plane with 46 passengers and crew on board crashed.
"The message was generated right before all contact was lost with the plane and it disappeared from radar screens," Russia's number two airline said in a statement.
The company also said there were indications that its plane exploded in the air.
"The wide distribution of large fragments indirectly confirms the conjecture that the plane broke up in midair because of an explosion," a company statement said.
Volga-Aviaexpress, a small regional carrier which owned the Tu-134, said the crew did not report any problems on board before the plane crashed with 43 passengers and crew.
Interfax news agency quoted an aviation source as saying the coincidence of both planes leaving from the same airport and disappearing at the same time would suggest it was "a planned action".
"In such a situation one could not exclude a terrorist act," the source was quoted as saying.
The crashes came against a backdrop of violence in Chechnya, where Moscow has been battling separatists for a decade. Rebels launched a major raid in the local capital Grozny last week.
Moderate Chechen separatists denied any role in the crashes.
Asked if his group was responsible for the crashes, Mr Akhmed Zakayev, a spokesman for Chechnya's separatist leader Mr Aslan Maskhadov, said in London: "Of course not."
"To us any form of terrorism is absolutely unacceptable. We have condemned it and continue to condemn it," he said.