A US military satellite detected a heat flash over the Sinai at the time a Russian airliner crashed, killing all 224 people aboard.
The Airbus A321 plunged to the ground on Saturday 23 minutes after taking off from the Red Sea resort of Sharm-el-Sheikh on a flight to St Petersburg.
Experts have said the fact that debris and bodies were strewn over a large area were signs the plane broke up in mid-air, a rare but not unprecedented event.
A US intelligence analysis has ruled out a missile strike, as claimed by a militant group affiliated with Islamic State, CNN reported.
But US officials told CNN and other US television networks that a military satellite detected a heat flash at the time of the crash, which points to a catastrophic event during flight, possibly the result of a bomb explosion although analysts were considering a range of possible causes.
Among other possibilities cited by the CNN report were the explosion of a malfunctioning engine, a fire caused by a structural problem on the plane or wreckage hitting the ground.
Investigators on Tuesday began examining the plane's two black boxes, one of which recorded on-board conversations and the other flight data, Egyptian civil aviation officials said.
Egypt's President Abdel Fattah al-Sisi, who has dismissed the missile strike claim as IS "propaganda," said it would take time to establish the cause of the crash.