A separate source, also not authorized to speak on the record, said that based on the facts so far, one of the working theories is that a bomb was planted at or near the fuel line or where it attaches to the engine, with the fuel burning off the explosive. This theory would explain the apparent lack of residue immediately found, the source says.
Fox News was told both scenarios point to an "airport insider."
"We always assumed Al Qaeda had this capability but now if ISIS has this capability, the threat to American airlines as well and our homeland, I think is very significant."
Fox News was told that Metrojet 9268 disintegrated approximately 23 minutes into the flight, and investigators are now focused on a “90-minute window” before the flight took off and who had access.
These new data points explain why investigators are interviewing ground crews and those with access to the departure lounge, as well as reviewing surveillance camera video. The review of surveillance images was first reported by ABC News.
The jet crashed in the Sinai Peninsula shortly after takeoff from Sharm el-Sheikh en route to St. Petersburg on Oct. 31. The crash killed all 224 people on board, most of them Russian tourists returning home.
The Defense Department was asked Tuesday whether C-4, an explosive provided to the Iraqi military by the U.S., had been obtained by ISIS.
"We don't have any reason to believe that any U.S. munitions were involved in whatever transpired with the crash itself," Pentagon spokesman Peter Cook said. "We're still waiting for the investigation to run its course."
A third data point that goes to the airport insider theory is that ISIS has claimed responsibility, though it has been reluctant to explain how it brought down the jet, leaving the airport mole in place.
The U.S. government and the intelligence community continue to emphasize that no firm conclusions have been reached about the cause of the explosion and crash.
The Associated Press contributed to this report.
Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent