MOSCOW, Aug. 27 -- The Russian government abruptly shifted gears Friday and concluded that at least one of the two planes that fell out of the sky almost simultaneously this week was blown up by terrorists, as a radical Islamic group claimed responsibility for the twin tragedies.
Investigators said they had discovered traces of a type of explosive previously used by Chechen bombers in the wreckage of a Tu-154 passenger jet that came apart in midair and plummeted to the countryside Tuesday night. Analysts were still studying fragments of the other plane, a Tu-134 airliner, which crashed about 500 miles away.
The discovery discredited the government's initial theory that human or mechanical error caused the crashes and renewed attention on Russia's war with separatist rebels in the restive southern region of Chechnya. Under the scenario being pieced together by Russian security services, a Chechen woman boarded each plane at a Moscow airport and triggered explosives in flight that brought down the jets within three minutes of each other, killing 90 people.
The crashes appeared to mark an escalation in a wave of terrorism that has unsettled countries around the world. For the first time since the Sept. 11, 2001, hijackings in the United States, terrorists apparently managed to infiltrate more than one plane in a coordinated attack, evading enhanced airport security.
The synchronized attacks, if carried out by Chechen separatists, would signal a change in tactics. Until now, the guerrillas have often struck soft targets, such as a subway cars, rock concerts and theaters, but never destroyed an airliner.
"They certainly wanted to demonstrate that they're very determined to continue their struggle with all possible means," said Sergei Arutyunov, a Chechnya scholar at Moscow's Institute of Ethnology and Anthropology. "Whether they really planned to use the planes as a 9/11 isn't clear, but it may have just been easier to blow them up."
The haunting echoes of the Sept. 11 attacks also raised questions about the involvement of outside Islamic fundamentalist groups. An extremist organization apparently affiliated with al Qaeda claimed responsibility for the Russian crashes on an Internet site, asserting that it had hijacked the planes in retaliation for what it called Russian brutality in Chechnya.
Officials said they could not determine the authenticity of the statement, which was signed in the name of the Islambouli Brigades. A group calling itself the Islambouli Brigades of al Qaeda claimed responsibility for an assassination attempt against Pakistan's prime minister-designate in July.
"Russia continues to slaughter the Muslims and will not stop unless a war starts where there will be bloodshed," the statement said, asserting that five attackers boarded each plane. "Our mujaheddin, thanks to God, were able to make the first strike, which will be followed by a series of other operations in a wave of support to our brothers, the Muslims of Chechnya and other Muslim areas that suffer the blasphemy of Russia."
In a cryptic addition, the statement said the attackers "succeeded despite the problems they faced at the beginning," a reference that was not explained.
Russia has linked the Chechen separatist movement to international terrorists, and Osama bin Laden has called the war in Chechnya a battlefield of a global holy war. But independent analysts have maintained that the Russian government exaggerates the relationship.
The government had been reluctant to tie the plane crashes to terrorism, a reticence critics attributed to an election Sunday in Chechnya in which the Kremlin wants to install its handpicked candidate to replace the region's former president, who was assassinated. But the weight of the evidence and universal criticism made it untenable to continue discounting terrorism.
At the crash site near the southern city of Rostov-on-Don, the Federal Security Service (FSB) said it had found traces of hexogen, a white crystalline powder also known as cyclonite or RDX. A powerful explosive often used by military forces, hexogen was also used in a series of apartment building bombings in 1999 blamed on Chechen separatists.
Russian investigators focused on two female Chechen passengers killed in the crashes whose families have not come forward to seek their remains. One was listed as S. Dzhebirkhanova on a passenger manifest for the Tu-154 that crashed near Rostov-on-Don, and the other was identified as Amanta Nagayeva, a passenger aboard the Tu-134 that fell in the Tula region about 100 miles south of Moscow.
Search crews in Tula found what they said they believed were Nagayeva's remains Friday, raising to 44 the number of people aboard the Tu-134 and the overall death toll to 90. Officials said part of her body was found in the tail section and other parts were found three miles away, suggesting she was at the center of an explosion.
Nagayeva was the last person to board the Tu-134, which was bound for Volgograd, buying her ticket just an hour before the flight. Dzhebirkhanova held a ticket for a different flight but switched hers at the airport at the last minute to the Tu-154, which was heading to the Black Sea resort of Sochi, according to Russian news accounts.
Security specialists said the two women or their accomplices possibly bribed someone to sneak their explosives on board. "It's not hard," said Sergei Goncharov, a former commander with the Alpha anti-terrorist commando forces. "The metal detectors will not detect it, and we can assume there was someone in security who was cooperating with them. Maybe they bought them off."
The Tu-154 sent out a distress signal and a hijacking alarm, but no voice communication, before it crashed, according to officials, while the Tu-134 issued no signals. Russian planes have general distress buttons in the cockpit as well as in the galley and elsewhere in the cabin. In the cockpit, behind locked, bulletproof doors, pilots can also send a hijack warning by punching in a special four-digit code.
A Russian pilot who regularly flies Tu-134 jets said he and his colleagues were told in training that as little as 400 grams of explosives would be enough to blow a hole the size of a soccer ball in the hull of the aircraft, causing violent decompression at the altitudes flown by those planes.
"If that happens, then it's very dangerous," said the pilot, who spoke on condition of anonymity because of the sensitivity of the situation. "A middle-aged man dies in two minutes maximum, and everyone becomes unconscious at once. That's why I think the crew didn't have any time to communicate anything to the ground."