Traces of explosive have now been found in the wreckage of both passenger jets which crashed earlier this week, Russian investigators say.
In both cases debris was scattered over a wide area
The two Tupolev airliners crashed almost simultaneously on Tuesday in southern Russia, hundreds of miles apart, killing 89 passengers and crew.
The FSB security service had already announced the discovery of traces of
hexogen among the remains of one jet.
Now it says the explosive has been found in the remains of the other.
After finding explosives traces on one plane, a Sibir Airlines Tu-154, on Friday, Russian officials described what had happened as a terrorist attack.
They also said the pilot had sent out a hijack alert just before the plane crashed, Russian media report.
FSB spokesman Sergei Ignachenko said on Saturday: "Additional examination of the fragments of the Tu-134
aircraft which crashed on Tuesday... has revealed traces of hexogen."
The two airliners took off within minutes of each other from Domodedovo airport in Moscow.
Reports say investigations are focusing on two women passengers, believed to be from the restive territory of Chechnya, where presidential elections are being held on Sunday.
Investigators say no-one has come forward to claim the women's bodies.
Officials had warned that Chechen separatist rebels could
resort to terrorism to try to undermine Sunday's voting.
Several suicide bombings in recent years have been carried out by Chechen women who lost husbands or brothers during the conflict in the southern republic over the past decade.
Russian officials have repeatedly contended that Chechen rebels receive help from foreign organisations, including al-Qaeda.
An obscure Islamist group has claimed responsibility for attacking both planes in a website statement.
The Islambouli Brigades said it would continue operations "until the killings of our Muslim brothers in Chechnya cease".
Russian officials have not commented on the claim. A group of the same name claimed recent militant attacks in Pakistan.
Hexogen, more widely known as RDX, was identified as the explosive in a series of apartment building bombings that killed some 300 people in Moscow and other cities in 1999.
The attacks were widely blamed on Chechen separatists.