MOSCOW -- As investigators race to establish why a Russian airliner crashed over Egypt's Sinai Peninsula on October 31, speculation mounts as to the cause.
Kogalymavia/Metrojet Flight 9268 is thought to have broken up in midair shortly after takeoff from the resort city of Sharm el-Sheikh, killing all 224 people aboard the Airbus A321-200 in Russia's worst-ever aviation disaster.
Kremlin spokesman Dmitry Peskov has said it is "too early" to rule out any theory, including a possible terrorist attack.
Here are some of the scenarios that Russian and other media suggest investigators are exploring:
An Egyptian affiliate of the Islamic State (IS) militant group claimed soon after the tragedy that it had downed the plane, but it offered no details. Russian Transport Minister Maksim Sokolov quickly dismissed the assertion, saying it "cannot be considered accurate."
A number of European and Middle Eastern airlines nevertheless announced on November 1 that they were changing their flight paths to avoid the airspace over the Sinai Peninsula.
Russia has emerged as a potential IS target since it launched a campaign of air strikes in Syria on September 30 against armed groups opposed to Syrian President Bashar al-Assad, including IS.
Egyptian authorities are meanwhile fighting Islamist militants in the Sinai Peninsula who pledged allegiance to IS in November 2014.
The Wilayat Sinai militant group operating in that area is not thought to possess the kind of surface-to-air missiles necessary to have struck Flight 9268, which was reportedly flying at an altitude of around 9,400 meters. The so-called MANPADS -- man-portable air-defense systems -- that the group is thought to possess are not capable of hitting targets at that altitude.
Aleksandr Smirnov, the deputy director for flight operations for Kogalymavia, which operates as Metrojet, told reporters on November 2 that the "only possible explanation for the breakup of an aircraft in the air could be a certain impact -- some mechanical or physical impact -- affecting the airliner."
The Metrojet Airbus A321 with registration number EI-ETJ that crashed in Egypt's Sinai Peninsula takes off from Moscow's Domodedovo airport on October 20.
Smirnov declined to elaborate, saying that is the work of investigators.
No evidence has emerged publicly to suggest that the disaster was caused by a bomb on board the plane.
The Kommersant daily quoted anonymous aviation experts as saying a bomb could have caused an explosive decompression that broke up the aircraft. The article drew a comparison to the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland.
Paul Hayes, the London-based director of air safety at the Ascend aviation-consultancy group, suggested that while the bomb theory was possible, it should be easily verifiable.
"They've got the wreckage; they should be able to rule that in or out quite quickly," Hayes said.
Kogalymavia has rejected the possibility of pilot error and stressed that pilot Valery Nemov had more than 12,000 hours of flight experience.
It noted that the plane's crew made no attempt to contact air-traffic control in their final moments, adding that would suggest the emergency situation came about suddenly.
"The reason the plane stopped flying and began to fall was probably the fact that, by that time, the plane had suffered significant structural damage preventing it from flying," said Kogalymavia's Smirnov. "This is the only explanation for a complete absence of attempts to establish communication or to report the emergency situation on board."
Earlier Tail Strike
There was initial speculation that the aircraft already suffered from a structural problem prior to takeoff.
In 2001, its tail section hit the runway in Cairo during touchdown -- an incident known as a "tail strike" -- requiring the plane to undergo months of repairs. Kommersant speculated the plane could have broken up in midair if it had a "fatigue fracture" on the fuselage that wasn't correctly repaired after the incident more than a decade ago.
In 1985, Japanese Airlines Flight 123 crashed after an explosive decompression blamed on shoddy repairs following a tail strike seven years earlier.
Hayes, however, called that theory "unlikely" in the Russian airliner's case.
"I don't want to put too much emphasis on this as a scenario," he said. "Yes, there have been aircraft that have crashed due to improper repair after tail strikes. But there are an awful lot of aircraft that have tail strikes and have been repaired and are flying quite happily with perfect repairs."
Kogalymavia has described its Airbus plane as in "excellent" condition. It was 18 years old, which the airline said means it was in "the middle of its lifespan."
Smirnov told reporters "we fully rule out a technical malfunction on board the aircraft."