NEW DELHI: As investigations into the crash of a Russian airliner over the Sinai peninsula, that killed 224 people on board on Saturday continue, UK investigators have said that it is possible that a bomb was placed in the aircraft’s hold prior to take-off.

The UK government, which suspended all flights to and from the Egyptian resort of Sharm el-Sheikh on Wednesday, felt terrorism was a likely cause of the crash after intercepting messages between militants in the Sinai peninsula.

The Metrojet Airbus A321 was flying from Sharm el-Sheikh to St Petersburg, when it crashed 23 minutes into the flight.

A report in the BBC refers to BBC security correspondent Frank Gardner saying that although British officials have not ruled out the possibility of a technical fault bringing the plane down, they think that is increasingly unlikely. Investigators in the UK's security service suspect someone with access to the aircraft's baggage compartment inserted an explosive device inside or on top of the luggage just before the plane took off, Gardner added.

Earlier in the week, a group claiming to speak on behalf of the Islamic State (also known as Daesh), posted an online statement saying: “Soldiers of the caliphate were able to bring down a Russian plane above Sinai province.”

At the time, officials warned against taking the claim seriously, pointing to the fact that militants could not have had the resources in terms of missiles to bring the plane down from the 30,000 feet that it was flying.

Russia’s transport minister, Maksim Sokolov, said the claim “can’t be considered accurate.” Mohamed Samir, Egypt’s army spokesperson, reiterated reasons for doubted the claim, saying: ‘“They can put out whatever statements they want but there is no proof at this point that terrorists were responsible for this plane crash. We will know the true reasons when the Civil Aviation Authority in coordination with Russian authorities completes its investigation. But the army sees no authenticity to the claims.”

Russian aviation regulator, Rosaviatsia, urged caution on arriving at theories for the crash till investigations were complete. “Until there is reliable evidence about the circumstances of what happened, there is no sense in putting forward and discussing any versions,” Rosaviatsia said.

Although the militant groups probably lack a missile capable of shooting down a plane from such a high altitude, the theory that a bomb was placed in the aircraft’s hold puts terrorism at the centre of investigations.

US President Barack Obama told a CBS radio station on Thursday that he thought there was "a possibility" that there was a bomb on the downed jet. "We're taking that very seriously," he said.

The Islamic State has been calling for attacks on Russian targets ever since Russia got involved in the conflict in Syria, on the side of Syrian President Bashar al Assad.

At this point, however, little is known about what caused the crash, and any theory is mere conjecture.

The crash continues a terrible spell in aviation, with several major crashes in 2014 and 2015. The two years have been some of the worst years yet in terms of casualties per accident.

Here’s a grim recap:

February 2014: a Nepal Airlines Twin Otter aircraft crashed into a mountainside near Sandhikhark, Nepal, on February 16, killing all 18 on-board.

March 2014: MH370 disappeared in March, and extensive search efforts since have yielded no sign of the plane. The flight’s disappearance is considered one of the biggest aviation mysteries, with evidence suggesting that the plane’s communication systems were “deliberately disabled,” leading to various theories including terrorist involvement or a potential hijack. With all passengers being cleared of suspicion, including two who were traveling on stolen passports, suspicion fell on the pilot, Zaharie Ahmad Shah, and co-pilot, Fariq Abdul Hamid, of MH370.

More significantly, the jetliner's data communications systems - the Aircraft Communications Addressing and Reporting System (ACARA) - were switched off, and investigators have pointed to the fact that someone with knowledge of the systems could have played a role, leading to doubt falling on the pilots themselves. Further, the last words spoken from the cockpit seem to indicate that nothing was wrong, with Hamid reportedly saying, “All right, good night" (later revealed to be goodnight Malaysian three seven zero) when Malaysian air traffic controllers informed them that control was being handed over to Vietnam. The plane never made contact with Vietnam, and investigators believe that the reassuring words were spoken at around the same time that the ACARA systems were turned off. So far, no evidence of the pilots’ involvement has been found -- and neither have any traces of the plane.

July 2014: MH17 was shot down over eastern Ukraine, killing all 298 people on board. The shooting-down escalated tensions between between Russia, which supports the pro-independence rebels in Ukraine’s east, and United States-led Western power that back the government in Kiev.

The tensions are linked to ambiguity regarding the circumstances that led to the plane’s crash, with Kiev releasing an audio of what it says are intercepted telephone conversations between rebels and Russian military intelligence officers during which the former admit to shooting down a plane. Leaders of the rebel Donetsk People's Republic denied involvement, saying instead that a Ukrainian air force jet had brought down the plane.

July 2014: TransAsia Airways ATR 72-500 crashed near Magong Airport in Taiwan on July 23, with 48 passengers and crew dying after the plane missed its first runway approach. Miraculously 10 people survived the crash.

July 2014: an Air Algerie MD-83 passenger aircraft dropped off the radar and crashed while flying over Mali, en route to Algiers from the Burkina Faso capital of Ouagadougou. All 116 occupants were killed.

August 2014: an Iranian-built Sepahan Airlines plane crashed shortly after takeoff from Tehran International Airport. 39 people died.

December 2014: QZ8501 disappeared en route to Singapore from Surabaya, Indonesia, with 162 people on board. Debris from the plane was located off the coast of Borneo soon after, with all passengers declared dead.

March 2015: A Germanwings plane with 150 people on board crashed over the French Alps on its from Barcelona to Dusseldorf on March 25. Evidence suggests that co-pilot Lubitz deliberately crashed the plane, killing himself and the 149 people on board.

June 2015: All 113 people on board a Hercules C-130 military cargo plane were killed as the transport plane crashed in a residential area of the Indonesian city of Medan. The plane hit two houses and a hotel before bursting into flames.