The 'nightmare scenario' for the Russian plane crash in Egypt is still very much in play

With US officials saying it's "99.9% certain" that the Russian plane that crashed in Egypt was brought down by a bomb, aviation authorities are having to face the prospect that the supposed attack was carried out by one of their own.

Evidence in the crash points to an inside job, aviation officials told ABC News. According to the report, authorities suspect the terror group ISIS, which has an affiliate in Egypt's Sinai peninsula, might have recruited an airport ramp worker to plant a bomb on the plane.

ISIS affiliates have released videos claiming responsibility for the crash, but the group's central leadership has so far remained silent. The affiliate says it brought down the plane in response to Russian airstrikes in Syria.

The strategic security firm The Soufan Group noted in a brief on Monday that "the nightmare scenario for aviation experts is the inside job; a trusted employee who can bypass the multiple layers of security enveloping international air travel." These types of attacks are "almost impossible to stop if the terror cell is not penetrated ahead of time," according to the group.

The lack of specifics from ISIS on how it supposedly downed the plane could also suggest an inside job.

"The absence of a martyrdom video suggests that the group had someone on the inside," The Soufan Group noted. "Until that person is arrested, the threat persists, leading the US and the UK to force immediate Egyptian security measures by going public with information they probably would have preferred to keep secret."

Airport officials in Sharm el-Sheikh, where Metrojet Flight 9268 took off on its way to St. Petersburg, are now investigating airport employees. The airport has also increased its security in response to the crash, according to ABC News. Employees will now be searched by X-ray scanners at the airport. Investigators are also questioning staffers as they look into the possibility of an inside job.

Rep. Adam Schiff (D-California), the ranking member of the House Intelligence Committee, told ABC's "This Week" on Sunday that "ISIS may have concluded that the best way to defeat airport defenses is not to go through them but to go around them with the help of somebody on the inside."

Rep. Peter King (R-New York), who also appeared on "This Week," noted that there are other airports in that region that have insufficient security.

But The Soufan Group said the alleged bombing of the Metrojet plane "might not herald a new age of similar attempts." That's because the location provided something of a perfect storm for such a possibility.

"If there was any place where the Islamic State could have pulled this off, it was in Sharm el-Sheikh," the group stated in a brief.

"Nowhere else is there an active and capable Islamic State affiliate in such proximity to a popular tourist destination and international airport. The Islamic State might be new to the Sinai, but its extremism and operatives have been in the region for decades. While the group's lone wolves are spread, in theory, across the globe, its affiliates are operating in conflict zones. ... That the Islamic State possibly penetrated the airport's security is as unsurprising as it is tragic."

Though it's possible lone wolves who work at airports in other countries could be recruited by ISIS (also known as the Islamic State), Sharm el-Sheikh's airport was especially susceptible to a terrorist plot given its location and relative lack of stringent security compared to other countries.

Michael Kofman, an expert on security and Russia and a public-policy fellow at the Wilson Center, told Business Insider last week that Egypt is "probably the best place to get a Russian airliner."

"If you look at where Russian tourists go, where else are you going to get a soft civilian target like that?" he said. "Turks are very geared up for terrorism. Egyptians are not. It's not that hard of a reach for somebody with a fairly sophisticated terrorist network or intelligence operation."

The evidence investigators have publicly disclosed so far remains vague. Egyptian officials have pointed to a strange sound in the final second of the recording from the plane's cockpit, and intelligence officials reportedly intercepted a calls between ISIS members in Egypt that warned of "something big in the area" before the plane crashed.

But even that doesn't guarantee the crash was an ISIS plot. The Soufan Group pointed out that terrorist groups "fill their communiqués with non-stop threats in hopes of overwhelming collective security measures while a real threat gathers momentum."

The Airbus plane crashed about 20 minutes after takeoff on October 31, killing all 224 people on board. Investigations are ongoing and no formal conclusions have been announced.