An “insider” ISIS……

Evidence now suggests that a bomb planted by the Islamic State (ISIS) militant group is the likely cause of last weekend’s crash of a Russian airliner over Egypt’s Sinai peninsula, U.S. and European security sources said on Wednesday.

Late Wednesday evening, CNN reported that US intelligence sources are now leaning toward the theory that an airport “insider” — possibly in airport security or a baggage handler — planted the bomb on board the plane. It could have had a simple timer or pressure gauge designed to detonate at a certain altitude.

Robert Baer, a former CIA officer and author, told CNN that he thought a plastic explosive was a likely tool used by the terrorists. He said it would have been quite easily for the terrorist group to have the expertise to design such a device. He said the technology was widespread in Baghdad among insurgents during the Iraq war.

“These people who make these bombs are very, very good,” Baer said.

It would be the most significant terrorist attack since 9/11, experts said.

Islamic State, also known as ISIS, which controls swathes of Iraq and Syria and is battling the Egyptian army in the Sinai Peninsula, said again on Wednesday it brought down the airplane, adding it would eventually tell the world how it carried out the attack.

The Airbus A321M crashed on Saturday in the Sinai Peninsula shortly after taking off from the resort of Sharm el-Sheikh on its way to the Russian city of St Petersburg, killing all 224 people on board.

The U.S. and European security sources stressed they had reached no final conclusions about the crash.

Britain on Wednesday cited the likely possibility of an explosive device as the cause of the crash, but made no mention of any group that may have been responsible.

“We have concluded that there is a significant possibility that the crash was caused by an explosive device on board the aircraft,” Britain’s foreign secretary, Philip Hammond, said after a meeting of the government’s crisis response committee chaired by Prime Minister David Cameron.

Hammond’s remarks came as Britain prepares to host a visit by Egyptian President Abdel Fattah al-Sisi this week.

Egypt, a close ally of the United States and the most populous Arab country, dismissed a similar claim of responsibility for the crash by Islamic State on Saturday.

“It is believed to be an explosion but what kind is not clear. There is an examination of the sand at the crash site to try and determine if it was a bomb,” said an Egyptian source who is close to the team investigating the black boxes.

“There are forensic investigations underway at the crash site. That will help determine the cause, to see if traces of explosives are found.”