Sinai Plane Crash Was 'Terrorist Act,' Russian Officials Say

MOSCOW — -- Russian investigators have said definitively that the cause of the Russian plane crash in Egypt that killed 224 people in October was caused by a terrorist bomb.

In response, President Vladimir Putin has vowed “retribution” against those responsible and ordered Russia’s military to intensify its operations against terrorist groups in Syria.

U.S. officials said Russia has already launched an aerial bombardment of Islamic State (ISIS) positions in Syria, hitting the group’s de facto capital Raqqa.

The Federal Security Service of the Russian Federation (FSB) says it will pay a $50 million reward for information leading to the arrest of those involved in the apparent plot.

The head of Russia’s FSB security service told President Vladimir Putin during a televised meeting that his experts had concluded that a homemade explosive device with the equivalent of about 2 pounds of TNT had been placed aboard the plane.

"We can say unequivocally that this was a terrorist attack," FSB Director Alexander Bortnikov told Putin.

The Metrojet plane, a Russian Airbus A32, had taken off from Sharm el-Sheikh and had been in the air for 22 minutes when it disappeared from radar screens and crashed into the desert.

Bortnikov said an analysis of passengers’ personal effects, baggage and pieces of the plane had shown traces of an "explosive materials of foreign production."

The FSB chief said the power of the bomb caused the plane to break-up in mid-air, "which explains why the fuselage of the plane was found across great distance."

Putin did not say who he believed to be behind the attack, but he strongly implied that Russia will now target the Islamic State, also known as ISIS or ISIL, which, along with its affiliate in Egypt, has already claimed responsibility several times for the crash.

"We will not wipe tears from our heart and soul. This will remain with us forever," Putin said. "We will look for them everywhere, wherever they may hide themselves, we will find them, at any point on the planet, and we will take retribution."

Putin said Russia will now intensify its military operations in Syria so that those behind the attack “will understand retribution is inevitable.”

The retaliation appeared to have already begun, with U.S. defense officials telling ABC News they had recorded a major bombardment of Islamic State positions by Russian forces in the past 24 hours; Russian ships fired 20 cruise missiles at Raqqa, Syria, and a dozen Cold War-era long-range bombers struck other ISIS strongholds.

The officials said Russia had informed the United States of the air operation before it began.

Russia has been bombing targets in Syria since late-September when it launched its campaign to support Bashar al-Assad’s government, saying it has hit hundreds of targets, including Islamic State fighters. In statements released claiming responsibility for the destruction of the Metrojet flight in Egypt, ISIS said it was a revenge for Russia’s strikes in Syria.