Russia said Friday terrorists were behind a passenger jet crash, as an Islamic group vowing support for Chechen rebels claimed responsibility for that attack and the downing of another airliner on Tuesday.

    Russia's Federal Security Service (FSB) said they had found traces of explosive materials in the wreckage of one of the two Russian planes, which crashed almost simultaneously Tuesday night after taking off from Moscow's Domodedovo Airport.

    "A study of the fragments of the Tu-154 aircraft discovered traces of an explosive substance," said FSB spokesman Sergei Ignachenko. He identified the materials as hexogen.

    Hexogen was identified by Russian authorities in 1999 as the explosive used in a series of apartment building blasts that killed around 200 people, attacks cited by Russian President Vladimir Putin as justification for making a war on the breakaway republic of Chechnya.

    Ignachenko said no similar evidence of terrorism had yet been found in the wreckage of the other plane, a Tu-134.

    "Information has been collected in the process of operational and search efforts to help determine the circle of people who may have been involved in the act of terrorism aboard the Tu-154," Ignachenko said.

    Russia's Interfax news agency quoted an unnamed official from the Chechen Interior Ministry as saying that a Chechen woman, identified as S. Dzhebirkhanova, was a passenger on the crashed Tu-154.

    Earlier reports have said Dzhebirkhanova was the only passenger on the crashed flight who had not been asked about by relatives.

    Akhmed Dakayev, chief of staff of Chechen Interior Ministry, told Interfax that besides Dzhebirkhanova, the ministry was also checking information about another Chechen citizen aboard the other downed airliner, which was en route from Moscow to the BlackSea resort of Sochi and crashed outside the city of Tula, 180 km south of Moscow.

    Officials said they were investigating possible links between the attackers and rebels in Chechnya.

    They said the remains of the female Chechen passenger were being closely examined for traces of explosive.

    A spokeswoman for the Ministry of Emergency Situations said thepassenger's remains were more badly damaged and more widely scattered on the ground than any others.

    Russia's aviation authorities said the Tu-154 airliner, which carried 46 people en route from Moscow to Volgograd and crashed inRostov region, sent hijacking signals before the crash.

    Meanwhile, the FSB declined to comment on reports posted on theInternet that an Islamist group has claimed responsibility for thetwin air crashes.

    "We are not going to comment on such statements. Moreover, it has not been established whether they are true or not," a FSB spokesman said.

    The Islamic group, calling itself the Islambouli Brigades, linked the crashes to events in Chechnya and hailed the downing ofboth planes as a first strike to stop Moscow's fight against separatists in Chechnya.

    "The Islambouli Brigades declares that our mujahedeen (fighters)have succeeded in hijacking two Russian planes," the group said inthe statement.

    The attacks "will be followed by a series of operations aimed to back and assist our brothers in Chechnya and other regions suffering from Russia," the group said.

    All 89 people on board the two Russian passenger jets were killed, raising fears of terrorist attacks ahead of Sunday's presidential elections in Chechnya, where separatist rebels have been blamed for a series of terror acts killing hundreds of people.

    Transport Minister Igor Levitin said Friday that the authorities had found a series of security faults in May this yearat Domodedovo airport.

    "Matters relating to security are more to do with airports thanwith airlines," said Levitin, who heads a commission charged with investigating the twin crashes.