Investigators in Russia say they've found evidence that explosives were used in the downing of at least one of two airliners which crashed within minutes of each other on Tuesday. The investigators say they found traces of explosive material in metal fragments from the larger of the two planes that crashed in southern Russia.

They identified the material as hexogen, the same chemical that authorities say was used in a series of apartment block bombings in Moscow in 1999.

Those bombings were blamed on Chechen separatist rebels and served as justification for then prime minister Vladimir Putin to send Russian troops back into the breakaway province.

The hexogen was reportedly found in the wreckage of the Tupolev-154 plane that crashed in the Rostov-on-Don region near the Black Sea. It went down almost at the same time a second, smaller airliner crashed south of Moscow. Eighty-nine people were killed in the crashes.

The crashes raised suspicions of possible involvement by Chechen separatists, who have been fighting Russian troops in a bid for independence for most of the past decade.

While a senior Chechen leader denied any involvement, an Islamic group used a website to claim responsibility for the crashes. The statement, which could not be verified immediately, purportedly came from a group identified as the Islambouli Brigades and said its fighters had acted in revenge for fellow Muslims in Chechnya.

Russian authorities declined to comment on the website claim.

The air crashes have raised tensions in Russia in the run-up to an election on Sunday in Chechnya to replace a president who was assassinated in a bomb attack in May.

Some separatist leaders have vowed to disrupt the election and even mount attacks in Russia proper.