An official explanation for the downing of a Russian passenger jet over Egypt has not been provided, but U.S. intelligence think it may have been bombed.

The jet came down over an isolated, mountainous area of the Sinai peninsula last weekend, killing all 224 people on board. Since the crash investigators have been studying black box data in order to determine the cause of the crash, and now it appears that terrorism is the most likely explanation, writes Barbara Starr for CNN.

ISIS Planted Bomb In Russian Jet: U.S. Intelligence

Intelligence suggests a bomb was planted  by ISIS on board Russian jet

An unidentified U.S. official told CNN that the most likely cause of the crash was a bomb planted by ISIS or an ISIS affiliate. The official pointed out that it is not an definitive conclusion at this stage.

“There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane,” said the official. Other officials also told CNN that intelligence analysis seems to suggest the presence of a bomb on board the plane.

“There had been additional activity in Sinai that had caught our attention,” the official said. The U.S. is monitoring ISIS activities, up to and including internal messages sent between members of the extremist group.

Investigation continues at Sharm el-Sheikh airport and crash site

The UK government has also expressed its belief that terrorism could have been to blame. It announced on Wednesday that it had “become concerned that the plane may well have been brought down by an explosive device.”

British officials did not explain how they had arrived at that conclusion, but a UK aviation team will travel to Sharm el-Sheikh airport to evaluate the facility and judge whether proper security is in place. The British transport minister claimed that an explosive device could have made it on board via “a person or in cargo.”

As a result of the ongoing investigation the UK government has delayed flights from Sharm el-Sheikh to the UK. So far the UK government has not changed its travel advice and is not advising Britons against traveling to the popular winter holiday destination of Sharm el-Sheikh.

Services to and from Sinai Peninsula restricted after crash

The Irish Aviation Authority has also banned Irish airlines from flying to and from Sharm el-Sheikh. An announcement on the IAA website claimed that the ban would be enforced “until further notice.”

So far U.S. authorities have not made any similar moves. White House Press Secretary Josh Earnest said that U.S, authorities did not make any similar announcements because there are no American carriers that maintain regular flights to and from the Sinai Peninsula.

“The situation is just different,” Earnest said Wednesday. “British authorities are taking the steps they believe are appropriate to safeguard the British traveling public. The United States has already taken steps that we believe are consistent with protecting American travelers overseas.”

Earnest did however tell the press that U.S. officials had been “in touch” with their Egyptian counterparts, but he refused to “get ahead of the ongoing investigation.”

Terror in the Sinai linked to Russian bombing campaign in Syria?

Following the crash it was speculated that terrorism may have been to blame due to the presence of ISIS-affiliated militants in the Sinai peninsula. One group claimed responsibility for the attack in the immediate aftermath of the crash, although there claims were largely dismissed.
There were echoes of the Ukraine attack in which Malaysia Airlines flight MH17 was brought down by a Russian-made Buk missile, but intelligence did not suggest that the Sinai militants had access to such sophisticated weapons.

Now it appears that terrorism may have been to blame, albeit using an explosive device carried by a passenger or a piece of luggage. If investigators conclude that this was in fact the cause of the crash it will cast serious spotlight on the failure of security procedures at Sharm el-Sheikh airport.

Major carriers including Air France and Lufthansa have canceled flights over the region, while other carriers have so far avoided rerouting their aircraft due to the extra fuel costs involved in flying longer routes.

Despite Russian President Vladimir Putin’s claims that the ongoing Russian bombing campaign did not increase the risk of a terror attack against Russia, there remains a possibility that ISIS or its affiliates targeted a Russian passenger jet in revenge. Intelligence analysts have expressed concerns that ISIS’ reach extends far beyond Syria and Iraq, with multiple militant organizations declaring allegiance to the group.

While the investigation is ongoing it sounds increasingly likely that militants bombed the Russian jet out of the sky . How will Vladimir Putin respond if those claims are proved to be true?