Three days after a Russian passenger plane crashed in Egypt killing all 224 people on board, the cause of the deadliest aviation disaster in Russia's history remains a mystery.

Russia was quick to deny a terrorism link after the Sinai Province of the Islamic State, the Egyptian arm of ISIS, claimed responsibility for downing the plane on Saturday. The official investigation is still underway.

But because the plane broke apart mid-flight, there is speculation that an explosion — possibly a bomb or sabotage — may be what caused it to crash only 20 minutes after takeoff.

Reports supporting that theory surfaced late Monday. Russia's Tass news agency reported that debris not belonging to the plane was found at the crash site, and a senior defense official told NBC News that an American infrared satellite detected a heat flash over the area where the plane crashed.

If a bomb was smuggled on board the jet, or if it was tampered with, that would have happened while parked at Sharm el Sheikh airport. With that in mind, security experts say Egyptian authorities are likely upping airport security and investigating the possibility of a breach.

Sharm el Sheikh Airport in south Sinai, Egypt, Saturday, Oct. 31, 2015.

"In order to sneak something onto the plane, this would probably require infiltration of the airport in some way, whether with a compromised security guard, or a disgruntled employee," said Zack Gold, a Middle East analyst and expert on security in the Sinai Peninsula.

He and other experts say the possibility of someone smuggling a bomb on board the flight exists, especially given the fact that Sharm el Sheikh's security has been breached before.

In May, flights were disrupted after a 29-year-old man crawled through a hole in a security wall and tried to open the door of a plane that was parked on the tarmac.

"Someone sneaking onto the tarmac is immediately more impactful on the need to raise security," said Gold.

Gold also pointed to the fact that Israeli airlines do not fly to Egyptian airports, because "Egypt doesn't provide them with what they need, in terms of security on ground," indicating broader security concerns.

Other experts say that security at Sharm el Sheikh's airport is as solid as at most other airports in North Africa and the Middle East. Besides standard screening equipment, the airport has a passenger screening canine team to detect explosives, and there are several security check points surround it.

Martin Reardon, vice president of security for Soufan Group in Doha, Qatar, said that a suicide bomber passing through the airport's security with explosives would be unlikely given the high level of security.

"You couldn't walk in with [improvised explosive device] hoping that someone screws up," said Gold. "There needs to be a hole you've identified, or an insider," such as flight crew, maintenance crew or someone else with security clearance and access to the tarmac.

Egyptian military personnel approach the wrecked Metrojet's tail in Hassana, Egypt, on Sunday, Nov. 1, 2015.

As an example of how that might happen, Reardon pointed to the 2010 case in which Saudi intelligence foiled an al-Qaida plot to blow up a cargo plane over the United States' eastern seaboard.

Then, the bomb was snuck on board the plane inside a small printer cartridge. It was disarmed before takeoff at Britain's East Midlands airport.

"It doesn't take a very large device to bring down an airplane," Reardon said when discussing how a small device would be easier for someone to smuggle on board an airliner. "A small bomb can blast a small whole in the plane that can cause its skin to rip off and break apart."

A Russian Ministry for Emergency Situations cargo plane carrying the bodies of the victims of Saturday's Metrojet crash in Egypt lands at Pulkovo airport outside St.Petersburg, Russia, Monday, Nov. 2, 2015.

Reached by Mashable by phone for comment, a spokesperson for the Sharm el Sheikh airport said only that "security operates as normal." When asked about the possibility of the airport's security being breached, the spokesperson simply hung up.