Suicide bombers destroyed two Russian domestic airliners Tuesday in precision attacks that killed 90 people, U.S. sources told ABC News.
Traces of explosives found in one of two downed Tupolev planes match explosives used in the 1999 bombing attacks of Moscow apartments by Chechen separatists, according to Russian officials.
"The results of the investigation have found hexogen," said Nikolai Zakharov, a spokesman for the Federal Security Service, known as the FSB.
Hexogen, a highly sophisticated explosive also known as RDX and cyclonite, is a clear, easily concealed, military explosive with very limited commercial use. Less than a pound can bring down a plane.
In what Russian sources are calling "Russia's 9/11," Sibir Airlines' Tu-154 and Tu-134 jets went down within 20 minutes of each other after taking off from the same Moscow airport Tuesday night.
Unidentified Chechen Women on Board
Russian officials suspect two female passengers of being the bombers. They seem to have been Chechen and are the only bodies not claimed by relatives or friends.
The ITAR-TASS news agency reported today that, according to air traffic controller sources, the Tu-154 signaled three times that it had been hijacked before it crashed.
According to the prestigious Kommersant newspaper, there were also irregularities about how the women got tickets on the doomed flights.
Sibir officials told the Kommersant the woman on the Tu-154 had originally bought a ticket for a flight the following day. But at the last minute, as the jet was boarding, she asked to change her ticket for a seat on the earlier flight, the newspaper reported.
She paid an extra 500 rubles ($16) to change her flight, the officials said.
‘Black Widows’ Take on Major Operations
Airline officials also said they could find no record of her providing a passport or other identification when she bought her ticket, according to the newspaper.
Both women were also given seats near the rear of the jets, where the explosions on each are believed to have occurred.
Based on the latest Russian reports, this is the presumed sequence of events:
9:45 pm: Tu-154 takes off. Woman suspect No. 1 was on board, bound for the Black Sea resort of Sochi.
10:31 pm: Tu-134 leaves for Volgograd, with woman suspect No. 2.
10:56: Tu-134 disappears from radar and crashes.
10:59: Tu-154 crashes, 500 miles away, after sending three hijack alarms heard by an air traffic controller.
Chechen female bombers — known as "black widows" in Russia — have recently begun participating in major militant operations in Russia.
The "black widows" first shot into the limelight during the deadly October 2002 Moscow theater siege and have been responsible for several attacks since, according to Russian officials.
Tuesday's aviation disaster came days ahead of crucial Chechen presidential elections set for this weekend, and Russian security forces have been on alert for terrorist attacks.
Web ‘Statement’ Claims Responsibility
The new findings follow a purported Web statement by an Islamist group taking responsibility for Tuesday's "operation."
In the statement, a group called the Islambouli Brigades said it had five mujahedeen, or holy fighters, on board each aircraft. It warned this act would be followed by others "until the killings of our Muslim brothers in Chechnya cease."
Russian security officials have refused to confirm or comment on the statement.
But a U.S. intelligence official said it's unclear if such a group even exists and warned that such statements on the Web should be taken "with a grain of salt."
Although it is not known if the Islambouli Brigades has any link to al Qaeda, a group calling itself the Islambouli Brigades of al Qaeda claimed responsibility for an attempt to assassinate a senior Pakistani official last month.
Lt. Khaled Islambouli was the leader of the group of soldiers who assassinated Egyptian President Anwar Sadat in Cairo in 1981.
ABC News' Mike Lee in Moscow, Richard Esposito and Dean Schabner contributed to this report.