(JTA) — Intelligence gathered by the United States suggests that a bomb caused the crash of a Russian passenger airplane over the Sinai.

U.S. news outlets including CNN and NBC quoted unnamed American officials as saying that investigators believe the plane may have been blown out of the sky.

“There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane,” an American official told CNN. NBC cited American officials as saying that Islamic State was responsible for the attack.

Among the signs that Islamic State is responsible, are internal messages of the terrorist group, which the U.S. monitors, according to CNN.

The Sinai affiliate of the Islamic State claimed responsibility for bringing down the flight on Saturday, saying it was in retaliation for Russian airstrikes on rebels in Syria’s civil war.

Russia began launching airstrikes on Syria in September saying it was coordinating with Syrian President Bashar Assad to combat terror groups such as Islamic State.

Britain on Wednesday grounded all flights to and from the resort town Sharm el-Sheikh, from where the flight left. British aviation experts made their way to the town for security checks before allowing British flights to return home.

A U.S. official told CNN that despite the fact that the Sharm el-Sheikh airport has lax security, “there is intelligence suggesting an assist from someone at the airport” in getting the bomb onto the plane.

Egyptian Civil Aviation Minister Hossam Kamel said officials have found no evidence to support the theory that a bomb caused the plane to crash, according to CNN.

Russian air transportation agency head Alexander Neradko, said Thursday it will take months for investigators to form credible theories.

Alexander Smirnov, the deputy general director of Kogalymavia airline, also known as Metrojet, told reporters on Monday in Russia that there were no technical failures on the plane, which he said was in excellent condition.

All 224 people aboard the plane were killed in Saturday’s crash, including a former program director for Hillel Russia.

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