U.S. and British intelligence officials say they believe a bomb was likely responsible for bringing down a Russian charter plane over Egypt's Sinai Peninsula that killed 224 passengers and crew on Saturday.
Investigators continue inspecting wreckage on the ground, as well as security at the Sharm el-Sheikh Airport where the plane took off, but many experts wonder how terrorists could have smuggled an explosive device on board in the first place.
Peter Lehr, lecturer in terrorism studies at the University of St. Andrews in Scotland, said the plane could have been brought down by an uncontained engine failure or "an insider threat, something like smuggling bomb on board. That's the most likely thing," he said.
Lehr said he believes the British government's decision to suspend all flights from the Egyptian airport to the United Kingdom indicates British officials have some kind of reliable information about the cause of the crash.
"You don't do that lightly because it costs many people lots of money," Lehr said. "There must be some smoking gun somewhere."
An estimated 20,000 British tourists remain at the resort city.
The Russian chartered Metrojet Airbus A321 was carrying vacationers from the Red Sea resort back to St. Petersburg when it crashed 20 minutes after takeoff from Sharm el-Sheikh on Saturday morning.
Lehr said that airport workers could have been bribed to smuggle luggage carrying a bomb, or that it could have been the result of a suicide bomber who wasn't checked properly at the gate. A group related to ISIS took credit for the bombing on social media, according to both CNN and NBC News.
But R. John Hansman, director of the International Air Safety Center at MIT, doesn't rule out a catastrophic engine failure as a cause of the crash.
"It's not easy to create a big-enough hole in an airplane to bring it down," Hansman told Discovery News. "In order to take the airplane down it would have to big enough to create a pressure wave or packed with enough shrapnel to create enough damage. Just putting holes in the fuselage is not enough to cause catastrophic failure."
Hansman said that each country enforces different safety standards for incoming flights. Planes coming directly to the United States, for example, have stricter rules for baggage and cargo inspection than on flights between other parts of the globe, he said.
Some nations allow cargo to be loaded on airplanes without direct inspections if the cargo companies that packed the material are pre-screened.
Investigators are looking at whether debris from the fuselage indicates a bomb blast from inside the craft, or shrapnel from the engine that penetrated from outside, according to Hansman. They can also determine the kind of a blast that occurred from examining the nature of wounds on the passengers bodies.
For his part, Lehr believes a possible terrorist bombing could be in retaliation for Russia's decision to begin air strikes in Syria last month, a military operation that Russia says targets ISIS.
"I think there's a very strong connection," said Lehr, who studies Islamic terrorism and its connection to aviation and maritime safety.