Two Russian passenger planes that crashed almost simultaneously last week were blown up in a "terrorist attack", according to investigators.
The FSB security service, which discovered traces of explosives in the wreckages of both aircraft over the weekend, said the planes were brought down by bombers on board.
"Today without a shadow of a doubt we can say that both airplanes were blown up as a result of a terrorist attack," Itar-Tass news agency quoted Lieutenant-General Andrei Fetusov as saying.
He said further analysis of the explosives would be needed to separate some of the components. But traces of hexogen, more widely known as RDX, had been found - an explosive used in other attacks in Russia blamed on Chechen rebels.
Transport Minister Igor Levitin said the crews reported no problems before the twin crashes, suggesting a bomb was triggered without advance warning.
He said an SOS signal sent from one of the planes could have been triggered by the force of the crash, not by the crew.
A Tu-154, en route from the capital Moscow to the Black Sea resort of Sochi, sent the SOS message just before it crashed last Tuesday. Less than four minutes earlier a Tu-134 came down on a flight from the same Moscow airport to Volgograd.
All 90 passengers and crew were killed.
Officials have refrained from blaming Chechens for the crashes, but theories in Moscow suggest that women believed to be Chechen took explosives on board and brought the planes down ahead of Sunday's election of a new president in the turbulent region.