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."
"If proven accurate, if ISIS did put a bomb on this aircraft which I believe to be true, it's a new chapter with respect to ISIS," Texas Republican Rep. Mike McCaul told Fox News. McCaul -- who receives regular briefings -- cannot discuss classified information, but said the Obama administration has consistently underestimated ISIS by emphasizing its focus on gaining territory, rather than expanding its reach to global plots.
