A U.S. official revealed that the latest U.S. intelligence suggests that the crash of a Russian passenger jet in the Sinai over the weekend was most likely caused by a bomb on the plane, leaving investigators focusing on ISIS operatives or ISIS sympathizers as the likely culprits.

Despite this declaration, the official stressed a formal conclusion has yet to be reached by the U.S. intelligence community, according to the Telegraph.

"There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," said the official who spoke on the condition of anonymity.

Since the Metrojet-operated Airbus A321 crashed in Egypt, killing all 224 passengers, many have questioned whether it was truly an accident or if something more sinister was at work, reported NBC News. Investigators fears may have come true Wednesday after a group claiming to be affiliated with ISIS took credit for the crash via social media.

As the investigation continues, the British government announced Wednesday that it had "become concerned that the plane may well have been brought down by an explosive device," according to CNN.

The British government did not explain how it came to that conclusion, but a U.K. aviation team is traveling to inspect the Sharm airport to see whether there were proper security measures at the airport and ascertain how an explosive device could have made it to the Russian airliner "including a person or in cargo," according to the British transport minister.

In the meantime, State Department spokesman John Kirby told reporters Wednesday: "We need to advise U.S. workers not to go to the Sinai."

No commercial U.S. flights operate over Sinai.