MOSCOW — Traces of explosives were found in the debris of the passenger jet that crashed in Egypt last month, the Kremlin announced Tuesday as it unveiled a $50 million reward in the case.

A senior Egyptian official also told NBC News that two employees at Sharm el-Sheikh airport had been detained in connection with the Metrojet bombing. Speaking on condition of anonymity, the source said the pair were being questioned. However, the country's interior ministry later issued a statement saying that no one had been "arrested."

Alexander Bortnikov, the chief of Russia's domestic security agency FSB, briefed a meeting of Russia's Security Council late Monday that a bomb equivalent to 2.2 pounds of TNT exploded on board the aircraft.

"You can definitely say that this is a terrorist act," he said.

"Retribution is imminent"

Following the briefing, President Vladimir Putin said "retribution is imminent." The Kremlin launched a massive overnight bombardment across Syria, including airstrikes on the ISIS stronghold of Raqqa, according to U.S. and Russian officials.

The FSB, which is the successor to the KGB, also offered the $50 million reward for information on who brought down the jet.

Russian President Vladimir Putin vowed to hunt the perpetrators, saying that "we will find them anywhere on the globe, and punish them."

He added: "We won't be wiping tears from our souls and hearts. It will stay with us forever. But that won't stop us from finding and punishing the criminals."

Putin also used the announcement to reaffirm Russia's commitment to airstrikes in Syria, where Moscow says it is bombing ISIS, the group that claimed responsibility for downing the plane.

"Our air operation in Syria will not just continue — it must be strengthened so the criminals understand retribution is imminent," Putin said, according to the Kremlin's statement.

According to U.S. military officials, Russia's overnight bombardment included fighter-bombers launched from its Syrian airbase of Latakia and cruise missiles fired from warships in the Caspian Sea.

Russian Defense Minister Sergei Shoigu told a briefing later Tuesday that warplanes delivered "a massive strike upon [ISIS] in Syria," according to the Interfax state-run news agency. Shoigu said the strikes hit Syria's Idlib and Aleppo provinces as well as ISIS positions in Raqqa and Der-ez-Zor.

The Metrojet crash left 224 people dead, most of them Russian tourists.

The Airbus A321 broke up in midair some 20 minutes into its journey from the resort town of Sharm el-Sheikh to St. Petersburg on Oct. 31.

U.S. officials have told NBC News earlier this month that intelligence intercepts picked up chatter between ISIS operatives boasting about taking down an airliner after the passenger plane crashed in the Sinai.