Four theories behind crash

Egyptian and international experts are investigating why a Russian airliner carrying 224 people crashed in the north of Egypt's Sinai Peninsula, killing all on board.

Bomb on board?

US and European security officials have said they have intelligence to suggest a bomb may have been planted by militants. On 4 November, Britain's Foreign Secretary Philip Hammond said: "We have concluded that there is a significant possibility that the crash was caused by an explosive device on board the aircraft." The UK and Ireland immediately halted flights to Sinai. An unnamed US official told the Associated Press news agency they had reached the "tentative conclusion", after intercepting communications, that an Islamic State (IS) affiliate in the Sinai peninsula had planted an explosive device on the plane. On Tuesday, US media reported that a military satellite had detected a "heat flash" over the Sinai at the time of the crash. Russian officials have already confirmed the plane broke up in mid-air.

Technical fault?

Egypt's Civil Aviation Minister Hossam Kamal said there had been no sign of any problems on board the flight, contradicting earlier reports that the pilot had asked to make an emergency landing after experiencing technical problems. But the Kogalymavia airline has insisted the 18-year-old plane was fully airworthy. Kogalymavia has said "external factors" were behind the crash - meaning the airline believes it was not a technical fault.

A girl cries during a funeral service for Nina Lushchenko, 60, a victim of the Russian MetroJet Airbus A321 crash, at a church in Velikiy Novgorod yesterday ahead of her burial. Russian airline Kogalymavia's flight 9268 crashed en route from Sharm el-Sheikh to Saint Petersburg on October 31, killing all 224 people on board. Photo: AFP

Human error?

The airline has said that the pilot - who reports identified as Valery Nemov - had more than 12,000 hours of flying experience, including 3,860 hours in A321s. Kogalymavia says it has ruled out "any mistake by the crew." But the aircraft's "black boxes" - the cockpit voice recorder (CVR) and the flight data recorder (FDR), both of which have been found - will provide investigators with detail on the last minutes of the flight and tell them whether any actions taken by the flight crew caused or contributed to the crash - which happened during fine weather.

Shot down by missile?

Suggestions that Flight KGL9268 was brought down by a missile fired by Islamic State militants have been met with scepticism by security experts. The jet was cruising well above the maximum range of any surface-to-air missile that the jihadists are thought to possess. Experts have also questioned the logic of why Islamic State's Sinai affiliate would risk inviting a massive international retaliation by such an action when its battle is primarily with the Egyptian state.