However, the investigation took on a new turn as forensic analysts focused on the presence of two Chechen women, each a passenger on the fated planes, the authorities said.
"Today we can say with the utmost certainty that both planes were destroyed as the result of terrorist acts," said security service spokesman Andrei Fetisov on Monday, quoted by the ITAR-TASS agency.
Fetisov added that investigators had concluded that the powerful explosive Hexogen was used in bringing one of the planes down.
A spokesman for the FSB intelligence service on Saturday said that traces of Hexogen had also been discovered in the wreckage of the second plane.
One plane left Moscow bound for the Black Sea resort of Sochi with 46 people on board. The other left the same airport headed for the southern city of Volgograd with 44 on board. All 90 people on both planes were killed.
Two Chechen women suspects
bought tickets at the last minute
Meanwhile, new details emerged on Monday about the two Chechen women - Nagayeva and S Dzhebirkhanova, as listed on their tickets - who are the focus of suspicion.
They lived in the same apartment in Chechnya, worked in the same market and, possibly, died within moments of each other on separate airliners that crashed.
How the explosive may have been brought on board the planes is still unclear. Investigators are still searching for clues about the two Chechens.
Nagayeva, 30, and the 37-year-old Dzhebirkhanova aroused investigators' suspicion because they purchased tickets at the last minute and were the only victims relatives had not inquired about after news of the crashes.
But Nagayeva's and Dzhebirkhanova's bodies have not yet been identified.
Nagayeva is said to have never
flown on an aeroplane before
Officials were considering two scenarios in which either Nagayeva and Dzhebirkhanova were indeed the bombers or their passports had been used by other women, the newspaper Izvestia reported, citing Chechen law enforcement officials. The newspaper also said Dzhebirkhanova's first name was Satsita and Nagayeva's, Amanta.
The two women who lived in an apartment in Grozny, Chechnya's war-shattered capital, were seen on 22 August leaving by bus from the town of Khasavyurt in the neighbouring province of Dagestan, the newspaper said.
They were believed to be en route to Baku, the capital of Azerbaijan, where they often bought clothes and other commodities to sell at the market in Grozny.
They were accompanied by two apartment mates and co-workers: Rosa Nagayeva - Amanta's sister - and Mariyam Taburova, the paper said.
Nagayeva was single, while Dzhebirkhanova had been divorced. Nagayeva's brother disappeared three years ago in Chechnya; the family believes he was abducted by Russian forces. A brother of Dzhebirkhanova, who had been an Islamic court judge under Chechen separatist president Aslan Maskhadov, was killed in 1998.
An unidentified Chechen Interior Ministry official told Izvestia that both women were "clean" of demonstrable ties to separatist fighters.
"Today we can say with the utmost certainty that both planes were destroyed as the result of terrorist acts"
Andrei Fetisov, spokesman, Russian security service
Neither Nagayeva's nor Dzhebirkhanova's relatives were aware of the two women engaging in any separatist activity, Izvestia reported. Nagayeva's mother said her daughter had never flown on an aeroplane.
According to the investigators, if the two women were in fact the bombers and had travelled from Grozny to Moscow, Taburova and Nagayeva's sister also could be suspects and be in the capital, Izvestia said.
Several human bombings in recent years have been blamed on Chechen women who had lost husbands or brothers in the war and chaos that has plagued the southern republic for most of the past decade.