Russia's security chief says an act of terror brought down the Russian A321 airliner in Egypt last month, killing all 224 people on board.
"Traces of foreign explosives" were found on debris from the Airbus plane, FSB chief Alexander Bortnikov told Russian President Vladimir Putin.
Mr. Putin vowed to "find and punish" those behind the attack over the Sinai Peninsula. A branch of so-called Islamic State said it downed the plane.
Nearly all the dead were Russians.
The bomb shattered the plane mid-air on 31 October.
Russia has offered a $50m (£33m) reward for information on the Sinai plane attackers.
Sinai Province, a branch of Islamic State (IS), said in a statement on 31 October that it had destroyed the plane because of Russian air strikes in Syria.