MOSCOW » A homemade explosive device brought down a Russian passenger plane over Egypt last month, the head of Russia’s FSB security service said today, telling President Vladimir Putin it’s now clear the crash that killed 224 people was a "terrorist" act and offering a $50 million reward for information leading to the arrest of those responsible.

The Islamic State group had already claimed responsibility for the attack, and Putin vowed to hunt down those who carried it out. Russian war planes struck Raqqa, the self-declared IS capital in Syria, and other targets today, the defense minister told Putin at a briefing later in the day.

The FSB appealed to the "Russian and international communities for cooperation in identifying the terrorists." It specified that the reward would be paid in dollars.

"According to our experts, a homemade explosive device equivalent to 1 kilogram of TNT went off onboard, which caused the plane to break up in the air, which explains why the fuselage was scattered over such a large territory. I can certainly say that this was a terrorist act," FSB head Alexander Bortnikov said.

He said tests showed the explosives had been produced outside of Russia, but gave no further details.

All of the people on board, most of them Russian tourists, were killed when the Metrojet Airbus 321-200 crashed over the Sinai Peninsula on Oct. 31, about 23 minutes after taking off from the Red Sea resort city of Sharm el-Sheikh. The plane was headed to St. Petersburg, where most of the passengers were from.

Egyptian authorities opened an investigation into a possible security breach or infiltration by militants of the airport staff soon after the crash, security officials said. The investigation has been focused on baggage handlers, their security overseers and also personnel involved in aircraft catering, according to the officials, who spoke on condition of anonymity because of the sensitivity of the case.

Egyptian Prime Minister Sherif Ismail said his government was informed only today of the results of the Russian investigation into the cause of the crash and he shied away from endorsing its conclusion that a bomb was to blame. Speaking in Sharm el-Sheikh following a Cabinet meeting there, Ismail said Egypt would "bear in mind" the Russian conclusion.

Aviation Minister Hossam Kamal, meanwhile, sought to cast doubt on the Russian declaration, saying the technical investigation into the crash by experts from Egypt, Russia, France and Ireland had not concluded its work.

Confirmation that a bomb was smuggled onto the aircraft would likely have a crushing impact on Egypt’s vital tourism industry.

Putin said Russia would be relentless in hunting down those responsible for the attack.

"There’s no statute of limitations for this. We need to know all of their names," Putin said. "We’re going to look for them everywhere wherever they are hiding. We will find them in any place on Earth and punish them."

The Islamic State group had claimed responsibility in written statements, as well as video and audio messages posted on the Internet following the crash. It said the attack was retaliation for Russia’s air campaign against IS and other groups in Syria, where Moscow wants to preserve the rule of President Bashar Assad.

At a briefing in Moscow today, Russian Defense Minister Sergei Shoigu told Putin that Russian bombers hit IS positions in Raqqa and other targets.

Putin said that Russia’s air campaign in Syria "should not only be continued but should be intensified so that the criminals realize that retribution is inevitable."

Putin’s meeting with Bortnikov and other senior officials was held late Monday, shortly after the president returned from meetings with other world leaders in Turkey. The leaders from the Group of 20 rich and developing nations had vowed to work together to combat the Islamic State group.

"In this work, including the search to find and punish the criminals, we are relying on all of our friends," Putin said. "We will act in accordance with the U.N. Charter’s Article 51, which gives each country the right to self-defense. Everyone who tries to aid the criminals should understand that they will be responsible for giving them shelter."

IS has warned Putin that it would also target him "at home," but did not offer any details to back its claim. IS has claimed responsibility for Friday attacks in Paris that killed 129 people and wounded 350 others.

Nataliya Vasilyeva in Moscow, Hamza Hendawi in Cairo and Robert Burns in Washington contributed to this story.