The bomb was likely planted by the terrorist group ISIS or an ISIS affiliate, CNN reported.

The intelligence is reportedly based partly on monitoring of internal ISIS messages.

Investigations are ongoing, however, and US officials haven't made any formal conclusions yet.

"There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," an unnamed official told CNN.

Officials are looking into whether ground crews, baggage handlers, or other ground staff could have planted a bomb on the plane, according to NBC News. A review of the passenger manifest and crew list didn't reveal anyone with ties to terrorism.

A White House spokesman didn't immediately respond to a request for comment on the report.

Separately, Britain said on Wednesday that the plane might have been brought down by an explosive device. The aircraft crashed in Egypt after taking off from Red Sea resort of Sharm el-Sheikh.