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Investigators probing what caused a passenger plane to crash Saturday in Egypt's Sinai Peninsula have turned their attention to who was on board — and who would have had access to the doomed jetliner before takeoff.

An airport source told NBC News that Egyptian security at Sharm el-Sheikh Airport has been focusing on who gained entry to the departure hall, was part of the catering and cleaning services, and had permission to be in the departure lounge.

Related: Satellite Shows Heat Flash When Russian Metrojet Plane Crashed, But No Missile

The Metrojet airliner en route from Sharm-el-Sheikh to St. Petersburg crashed in the Sinai Peninsula on Oct. 31, 2015, killing all 224 people on board. MAXIM GRIGORIEV / RUSSIAN EMERGE / EPA

The Metrojet-operated Airbus A321, which was headed for St. Petersburg, was carrying 224 people on board, mostly Russian vacationers visiting the Red Sea resort. No one survived.

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The intrigue over the Russian charter jet's final moments has only deepened as aviation experts float two theories: Either there was a bomb smuggled on board or some sort of catastrophic mechanical failure occurred in mid-flight.

American intelligence sources told NBC News that none of the passengers or crew members were found on any U.S. terrorism databases.

U.S. officials on Monday also dismissed earlier speculation that a missile could have struck the airliner since infrared satellite imagery showed no possible heat trail that would have come from a launched missile. Similarly, Russian media reported there were no signs of blast-related trauma found on the recovered bodies.

Related: What Could Cause a Plane to Break-up in Midair?

Metrojet officials said at a news conference that the pilots never dispatched that they were in distress and made no effort to contact Egyptian traffic controllers in the event the crew saw something headed the plane's way.

Russia's Interfax news agency, meanwhile, claimed that sounds taken from cockpit recordings from the airliner's black boxes were "uncharacteristic of a standard flight."

Some analysts have said a 2001 accident in which the Airbus A321's tail section was damaged when it smacked into the tarmac on landing could have contributed to a longstanding structural problem.

But the plane, which was originally registered in Ireland, was repaired. Irish authorities said that as of last May, the plane's certification was in order.

Capt. Mike Vivian, a U.K.-based aviation and safety analyst, said that if the aircraft cabin was physically compromised in any way, that would have proved disastrous.

"If you have a pressurized environment such as an aircraft cabin, obviously a fracture is very serious and could potentially bring the aircraft down," he said.