MOSCOW - One of two Russian airliners that crashed nearly simultaneously was brought down by a terrorist act, officials said today, after finding traces of explosives in the plane's wreckage. A Web site connected to Islamic militants claimed the action was connected to Russia's fight against Chechen separatists.

The planes, with 90 people aboard, went down within 20 minutes of each other Tuesday night.

"According to preliminary information, at least one of the air crashes ... has been the result of a terrorist act," a spokesman for the Federal Security Service, Sergei Ignatchenko, the ITAR-Tass news agency reported.

No results from the investigation of the other crash - a Tu-134 with 44 aboard that went down about 120 miles south of Moscow - have been announced.

Another security service spokesman, Nikolai Zakharov, said the explosive found in the remains of a Tu-154 that carried 46 people appeared to be hexogen - an explosive officials said was used in the 1999 apartment bombings that killed some 300 people in Russia, an attack blamed on Chechen separatists. The Tu-154 crashed en route to the Black Sea resort city of Sochi.

Despite the suspicious timing of the crashes and the fact they took place five days before an election in Chechnya opposed by separatists, Russian officials had kept open the possibility they were caused by bad fuel or human error.

A Web site connected to Islamic militants published a statement on today - signed the "Islambouli Brigades" - claiming responsibility for the crashes. A group with a similar name has claimed responsibility for at least one other attack, but the authenticity of today's statement could not immediately be confirmed.

The statement said five "mujahedeen" - holy fighters - were aboard each plane.

The Federal Security Service declined to comment on the statement.

Russian officials have contended that the rebels fighting Russian forces in Chechnya for nearly five years receive help from foreign terrorist organizations, including al-Qaida. Today's claim did not refer to al-Qaida, but a group called "the Islambouli Brigades of al-Qaida" claimed responsibility for last month's attempt to assassinate Pakistan's prime minister-designate.

Russian officials, meanwhile, said they were investigating two female passengers - one on each plane - with Chechen names. The two were the only passengers whose relatives did not contact authorities, officials said.

Female suicide bombers with alleged Chechen connections have carried out attacks in Moscow.

Paul Duffy, a Moscow-based aviation expert, told Associated Press Television that he found it "hard to believe" that five attackers were aboard each plane, "but there is no doubt that they had one at least on each aircraft."