Last Saturday, a Russian jet carrying 217 passengers crashed in Egypt’s Sinai Peninsula. The Metrojet flight was making its way to St. Petersburg from a popular Red Sea resort when it went down in a remote, mountainous region of the Sinai.

Almost immediately, two European airlines suspended all travel over the region where the jet crashed—and for good reason. According to the Associated Press, the Hassana area where the wreckage was found has played host to clashes between Egyptian forces and a group of Islamic militants who recently pledged their loyalty to the Islamic State.

A local ISIS affiliate declared responsibility for the crash (now, attack), but the claim was quickly scuttled by both Egyptian and Russian officials. Now, however, US intelligence reports have revealed that it is likely ISIS or one of its affiliates planted a bomb on the plane.

Via CNN:

“There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane,” the official said, stressing that no formal conclusion had been reached by the U.S. intelligence community.

The assessment was reached, the official said, by looking back at intelligence reports that had been gathered before Saturday’s plane crash and intelligence gathered since then. The United States did not have credible or verified intelligence of a specific threat before the crash. However, the official said, “there had been additional activity in Sinai that had caught our attention.”

Egyptian authorities haven’t publicly responded to reports on U.S. intelligence. Since the crash, they’ve downplayed the possibility that terrorism could be involved.

More from CBS News:

Russian officials, as well as Metrojet, have insisted that the plane crashed happened due to an “external impact.” U.S. satellite imagery detected heat around the jet just before it went down, but officials say that heat signature could have come from any number of things (think an engine exploding)—not just a bomb blast.

According to terrorism analyst Paul Cruickshank, “This would be the most significant terrorist attack since 9/11, there’s no doubt about that, with the Russian involvement in Syria. If ISIS really was responsible for this, this will turbocharge their popularity in the global jihadi movement.”

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