Helicopter flies over wreckage of Metrojet Flight 9268
Helicopter flies over wreckage of Metrojet Flight 9268Reuters

US officials investigating the crash of a Russian airliner over the Sinai Peninsula now believe ground crew at Sharm al-Sheikh airport were responsible for smuggling aboard an explosive device which downed Metrojet Flight 9268.

According to MSNBC news, US officials still view a bomb as the most likely explanation for the crash, but say that all air crew and passengers had been screened and none of them had any known connections to terrorist or extremist groups - raising the prospect that someone else with access to the plane planted the bomb.

That being the case, it would not have taken a particularly sophisticated device - for example, advanced surface-to-air missiles, a theory debunked shortly after the crash itself - to down the aircraft. A relatively rudimentary bomb would simply need to be well-concealed enough to pass airport security. 

According to numerous travelers who passed through the airport recently, security was shambolic-to-nonexistent.

"It was dreadful," one woman told Sky News. "Staff actually had to be woken up to scan hand luggage." 

"When I was at Sharm el-Sheikh the belt at security was broken so that the bags couldn't go round. Hundreds of us were told to just leave the bags in a big pile in an insecure area and move on through," another one said.

Egyptian and Russian officials continue to play down suspicions of terrorism, even as an increasing number of other countries' airlines suspend flights to and from Sharm al-Sheikh.

224 people died when the Metrojet plane crashed on Saturday.

The investigation into the crash is still ongoing.