Egyptian officials reportedly detained 17 airport employees Tuesday, hours after the head of Russia's security service said that the crash of a passenger plane over the Sinai Peninsula last month that killed all 224 people on board was an act of terrorism.

Reuters, citing two security officials and an employee of the airport in the Red Sea resort town of Sharm El-Sheikh, reported that 17 people were being held, but only two were strongly suspected of assisting whoever planted the bomb on the Metrojet airliner Oct. 31.

One security official told Reuters that closed circuit television showed a baggage handler carrying a suitcase from an airport building to another man, who was loading luggage onto the plane. Reuters also reported that security forces were looking for two men accused of leaving a luggage-scanning machine briefly unattended while passengers were boarding the plane.

Earlier Tuesday, Alexander Bortnikov, the head of Russia's FSB security service, told Russian President Vladimir Putin that a homemade explosive device blew up on the plane shortly after takeoff. Bortnikov added that traces of explosives were found in debris at the crash site.

Bortnikov told reporters that the device was equivalent to more than two pounds of TNT, "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."

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

ISIS has claimed responsibility for bringing the Russian plane down 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 ISIS and other groups in Syria, where Moscow wants to preserve the rule of President Bashar Assad. 

A $50 million reward has been promised for information that leads to the arrest of those responsible for the bombing, according to Russia's Interfax news agency. Russian President Vladimir Putin ordered special services to focus on finding those responsible for bringing down the plane.

"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 Metrojet airliner, an Airbus A321-200, crashed shortly after taking off from Egypt's Sharm el-Sheikh International Airport en route to Saint Petersburg. Both U.S. and British intelligence indicated terrorism as the cause in the immediate aftermath of the crash, despite early denials from the Egyptian authorities.

In Cairo, there was no immediate comment on the news from the Egyptian government.

Egypt had resisted British and U.S. assertions that an explosive device was the likely cause of the Russian plane's crash. Later, government officials and the pro-government media shifted their focus away from the cause of the crash to speculating on what they called a Western conspiracy against Egypt and the crushing impact of the crash on the country's vital tourism industry.

The U.K., Russia and several other countries suspended flights to and from the popular tourist destination, stranding thousands of vacationers in the Red Sea resort, and forcing airlines and tour operators to scramble to get travelers back to their home country.

Putin said Tuesday 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."

He instructed the Defense Ministry and General Staff to present their suggestions on how Russia's operation in Syria could be modified.

"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."

Russia made the announcement the day after meetings with other world leaders in Turkey, where they vowed to work together to combat ISIS.

The group warned Putin that they would also target him "at home" but did not offer any details to back its claim. While releasing specifics would add credibility, the group may be withholding either because its claim is false, or because doing so would undermine plans for similar attacks in the future -- or because the aura of mystery might deepen its mystique among die-hard followers.

ISIS has also claimed responsibility for Friday attacks in Paris that killed 129 people and wounded 350 others.

The Associated Press contributed to this report.

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