Two Russian passenger planes that crashed almost simultaneously last week killing 90 people were blown up in a "terrorist attack" but not by hijackers, investigators said yesterday.

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 out 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, ordered by President Vladimir Putin to head a commission investigating the crashes, said the crews reported no problems before the twin crashes, suggesting a bomb was triggered without advance warning.

"From the information gained from the flight recorders, there is no reason to talk about a hijacking," Mr Levitin told local television.

Mr Levitin said an SOS signal sent from one of the planes could have been triggered by the force of the crash, not by the crew.

"It was activated but it happened almost at the moment the aircraft was destroyed, in fact a tiny fraction of a second later," he said. "This gives us the conditions to think that it was triggered not by the crew but maybe by a short-circuit when the aircraft was breaking up."

A Tu-154, en route from the capital Moscow to the Black Sea resort of Sochi, sent the SOS message just before it crashed on 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. Some families were still waiting to receive their relatives' remains. Hundreds of relatives attended funerals in Volgograd and Sochi at the weekend.

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.

Chechen rebels have staged spectacular attacks to press for independence. But moderate separatists have denied any link to the crashes and accuse Russian authorities of spreading lies.

Alu Alkhanov, who is backed by the Kremlin and has been marked for death by Chechen rebels, won the election in a landslide and vowed to bring peace to the region.

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