MOSCOW - Russian airlines will be required to document passengers more thoroughly following this week's twin plane crashes, a news agency reported Saturday, as the discovery of explosives stoked speculation that the disasters were linked to bloodshed in Chechnya.

The Federal Security Service, or FSB, declared at least one of the crashes an act of terrorism Friday after investigators found traces of the high explosive hexogen in the wreckage of a Tu-154 jet.

Results from investigation of another plane that went down almost simultaneously on Tuesday have not been announced.

While announcing the discovery of hexogen traces, officials kept silent about who they think is behind the attack, which came amid official warnings that Chechen rebels might flex their muscle in a bid to discredit Kremlin-sponsored presidential elections.

A Web site connected to Islamic militants, however, claimed the crashes were retaliation for Russia's protracted civil war in Chechnya, and Russian officials said they were investigating the backgrounds of two female passengers with Chechen surnames - one on each of the planes.

A Chechen village leader, Dogman Akhmadov, told the newspaper Izvestia that the brother of one of the suspected women had disappeared three or four years ago and was believed to have fallen victim to Russian forces who are widely accused of civilian abductions and summary executions in the southern republic.

Several suicide bombings in recent years have been blamed on Chechen women who lost husbands or brothers in the war and chaos that have plagued the region for most of the past decade.

Both women had booked tickets on the flights at the last minute and were the only victims whose relatives have not contacted authorities, officials said. One of the women gave only her surname and first initial in booking the ticket, according to reports.

President Vladimir Putin made no public comment on the discovery of traces of explosives, apparently not wanting to highlight an embarrassing security lapse, three days after what one major newspaper called Russia's Sept. 11.