Russian investigators have confirmed that explosions caused two almost-simultaneous plane crashes last week.
Russian authorities have vowed to improve aviation security
"Both planes were destroyed as the result of terrorist acts," spokesman Lt Gen Andrei Fetisov told the Itar-Tass news agency.
Investigators say traces of explosive were found in the remains of both jets.
The two Tupolev airliners crashed within minutes of each other in southern Russia, hundreds of miles apart, killing 89 passengers and crew.
Transport Minister Igor Levitin told Russian television that the crews reported no problems, suggesting a bomb was set off without advance warning.
A distress signal sent from one of the planes could
have been triggered by the force of the crash, he added.
Reports say investigations are focusing on two women passengers, believed to be from the restive territory of Chechnya, where presidential elections were 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.
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.