Russian officials said Friday they detected traces of a high explosive in the wreckage of one of two crashed jetliners, branding it the work of terrorists, while an Islamic group claimed its suicide attackers brought down both planes because of the war in Chechnya.
At least one crash was "the result of a terrorist act," a spokesman for the Federal Security Service, Sergei Ignatchenko, told the ITAR-Tass news agency. In Washington, White House spokesman Scott McClellan said evidence was growing that both crashes "were acts of terrorism."
Russia's leaders didn't speculate publicly about who might be responsible for the crashes, which killed a total of 90 people. But officials confirmed they were looking into the backgrounds of two female passengers with Chechen names who booked tickets on the doomed flights at the last minute and who were the only victims whose relatives did not contact authorities.
CBS News Correspondent Richard Roth reports women have been playing a major role in Chechen terror -- from the suicide-belted women who helped take hostages in a Moscow theater two years ago to the Chechen woman who blew herself up at a rock concert last summer.
In addition, the explosive hexogen detected on plane parts is the same type that Russian officials reported being used in earlier attacks by Chechen separatists.
On an Internet site connected to Islamic extremists, a statement from a militant group said the planes were attacked in retaliation for Russia's war in the predominantly Muslim region of Chechnya and warned it was only the first in a series of planned operations. There was no way to check the claim's authenticity.
The official announcement that terrorists had struck Russia's civil aviation industry — an essential transport sector for this vast nation — prompted only a low-key response. The government avoided drastic measures such as closing airspace or grounding flights, and President Vladimir Putin made no public comment on the evidence of explosives.
Analysts said the government was trying to avoid an embarrassing admission that Chechen separatists had succeeded again in striking at Russia — just days before a Sunday election in Chechnya to replace the small republic's assassinated pro-Kremlin president.
A Chechen connection to the crashes probably would intensify the Kremlin's already hard line in refusing to negotiate with the separatists, although it also would emphasize the failure of the military and security services to defeat the rebels.
"Here's the answer to how effective our politics in Chechnya have been," Russian legislator Vladimir Ryzhkov was quoted as saying in the newspaper Novaya Gazeta.
Security analyst Andrei Soldatov said a Chechen connection could bring more suffering to that region, where Russian troops have been widely accused of abusing and abducting civilians.
"The government will now be able to say that the fight against separatists in Chechnya comes under the roof of international terrorism. As soon as they say that, you can forget about human rights in the region," Soldatov said.
A spokesman for the Federal Security Service, Nikolai Zakharov, announced that experts had found traces of hexogen, a high explosive, in the scattered remains of a Tu-154 jetliner that went down late Tuesday with 46 people aboard.
The government previously said hexogen was used in 1999 bombings at Russian apartment buildings that killed some 300 people and were blamed on Chechen separatists. Those bombings led in part to Putin's decision to send Russian troops back into Chechnya, which had been semiautonomous after previous fighting.
CBS News consultant Paul Duffy, a Moscow-based pilot and former head of the civil aviation authority in Ireland, says very little hexogen would be needed to down an aircraft.
"Maybe a quarter pound to half a pound — If you're sitting beside the fuselage skin, that'd be all it would take," Duffy said.
Both planes took off from Moscow's Domodedovo airport, one of Russia's most modern and sophisticated. It was not immediately clear how airport security systems could be circumvented to smuggle in explosives.
Duffy tells CBS News the planes were parked side-by-side and serviced by the same crew. The same clerk checked in the passengers to both flights.
Authorities did not discuss what searchers had found at the second crash site, of a Tu-134 airliner with 44 aboard. Officials had said previously that 43 people were on the plane.
A Web site connected with Islamic militants published a statement signed the "Islambouli Brigades" claiming responsibility for both crashes. "Russia's slaughtering of Muslims is continuing and will only stop when a bloody war is launched," it said.
A group with a similar name, "the Islambouli Brigades of al Qaeda," claimed responsibility for last month's attempt to assassinate Pakistan's prime minister-designate, although Friday's statement did not mention al Qaeda.
Russian officials have repeatedly said that the rebels who have been fighting Russian forces in Chechnya for nearly five years receive help from foreign terrorist groups, including al Qaeda.
The statement released Friday said five "mujahedeen" — holy fighters — were aboard each of the crashed planes.
Paul Duffy, a Moscow-based aviation expert, told Associated Press Television he was skeptical that so many attackers could get on each plane. "But there is no doubt that they had one at least on each aircraft," he said.
Officials confirmed Russian media reports that two women passengers were being investigated.
Amanta Nagayeva, a Chechen native, bought her ticket one hour before the Tu-134 departed, NTV television said. It said she was seated in the rear of the plane near the tail, which was found severed from the rest of the fuselage. Search crews found fragments of her body Friday, Russian news agencies said.
The other woman, S. Dzhebirkhanova, originally was scheduled to fly to the Black Sea resort of Sochi, where Putin was vacationing, on a Tuesday morning flight, but she changed her ticket at the last minute, according to Gazeta.ru, an Internet news site.
Female suicide bombers with alleged Chechen connections have carried out several attacks in Moscow, including last year's double bombing at an outdoor rock concert and another blast outside a hotel adjacent to Red Square.
Representatives of Chechen rebel leader Aslan Maskhadov earlier in the week denied any connection to the plane crashes. But Maskhadov, who led Chechnya during its 1996-99 de-facto independence, is believed to control only a small portion of separatist fighters.