An Egyptian airport "insider"—possibly an airport security officer or a baggage handler or a food caterer—could have placed a bomb, possibly a plastic explosive to avoid detection, inside the Russian jetliner that crashed over the Sinai peninsula in Egypt on Saturday, killing all 224 people on board.

This likely scenario was described on Wednesday by U.S. intelligence sources as evidence grew that the disaster was a terrorist act possibly committed by the Islamic State (ISIS) jihadist group.

If the assertion is confirmed, it could be considered as the most significant terrorist attack since 9/11, sources said.

U.S. intelligence sources are now focusing on the theory that somebody placed the bomb on board the Airbus A321M, CNN reported on Wednesday evening. The sources said the bomb 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 a plastic explosive was the likely device used by the terrorists, noting that terrorists would not find the device difficult to make or purchase since the technology was already well-known among insurgents during the Iraq war.

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

Britain also backed the theory that a bomb caused the crash of the Russian jetliner.

"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 on Wednesday after a meeting of the government's crisis response committee chaired by Prime Minister David Cameron.

An Egyptian source also said they believe there was an explosion on board "but what kind is not clear, adding that "there is an examination of the sand at the crash site to try and determine if it was a bomb," according to the Daily Express.

ISIS repeated on Wednesday that it carried out the terrorist attack on the Russian plane, 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.

As the bodies of the victims arrive back in Russia on Wednesday, experts said the state of their remains suggested there was an onboard explosion.

Another suggestion that pointed to a blast from inside the plane came in the form of an infra-red satellite, which found a heat flash on the Russian plane.

The managers of the catering company which delivered food onto the doomed plane on the morning of the crash have already been interrogated by Egyptian authorities.

After being wheeled on board, food trolleys are stowed just in front of the rear pressure bulkhead at the back of the aircraft—the part that was apparently blown off before the crash.

Russian officials are also leaning on the bomb theory, with one aviation official saying their investigation is looking into the possibility of an object stowed on board that caused the disaster.

"There are two versions now under consideration: something stowed inside (the plane) and a technical fault. But the airplane could not just break apart in the air—there should be some action. A rocket is unlikely as there are no signs of that," the Russian official said.

Missile strike ruled out

Security experts and investigators have said the aircraft could not have been possibly downed by a missile strike since Sinai-based Islamist militants are not believed to possess the weapon that could shoot down a jet flying above 30,000 feet.

Investigators have already checked the contents of the flight data recorder, one of two so-called black boxes recovered from the Russian plane, Egypt's Civil Aviation Ministry said.

The ministry said the second black box, which contains the cockpit voice recorder, was partially damaged, adding that much work still needs to be done to be able to extract data from it.

20,000 Britons stranded in Sinai city

Meanwhile, more than 20,000 Britons were stranded in Sharm el-Sheikh, the Egyptian resort city in the Sinai Peninsula where the ill-fated Russian plane took off, after Britain cancelled all British flights to and from the city, the Express reported.

Downing Street grounded all passenger aircraft flying out of Sharm el-Sheikh, fearing that the terrorists who brought down the Russian plane could still be in the region and could target other passenger aircraft.

RAF jets are already at the airport preparing to bring back some of the 20,000 Britons stranded in the region.

In announcing the move, Hammond said the government made the decision to halt all flights to and from Sharm el-Sheikh airport to Britain after receiving "sensitive intelligence."

ISIS to crash doubters: 'Die from your frustration'

The ISIS released a second message on Wednesday claiming responsibility for the Russian plane crash – but said it may never reveal how it downed the jet, the Express reported.

The speaker said in Arabic: "We say to the deniers and the doubters: Die from your frustration. We, with God's grace, are the ones who brought it down, and we are not obliged to disclose the mechanism of its demise.

"So go to the wreckage, search, bring your black boxes and analyse, give us the summary of your research and the product of your expertise and prove that we did not bring it down or how it came down.

"We will disclose the mechanism of its demise at the time that we want and in the way that we want."