All 224 passengers and crew members on board Kogalymavia Flight 9268 were killed on Saturday after the airbus fell apart mid-air and crashed over Egypt's Sinai Peninsula just 23 minutes into its journey back to St. Petersburg.
There is still no clarity as to what happened on board the Russian plane. Was it a missile that brought down the Airbus? Who fired the missile? Was it Isis? or was there a bomb on the plane?
To find answers to these questions, since Saturday, investigators from several countries have been sent to the crash site.
The investigators have found both the black boxes. While the personnel are recovering the plane parts and the bodies of the victim, a full report on the actual cause of the crash could easily take months.
Here, we have examined a few theories to understand, what really caused the Russian plane crash.
Isis shot down the plane: The aircraft, which carried the brand name "Metrojet", took off from the Red Sea resort of Sharm el-Sheikh at 5.51 am and was headed to St Petersburg when it disintegrated at 31,000 feet.
The Islamic State (Isis) terrorists were quick to take credit for the crash, claiming that Sinai wing of Isis shot down the Russian plane. Initially many believed it since Russia is carrying out a massive air campaign against Isis in Syria.
Moscow, however, later refused to accept the assertion, as shoulder-fired rockets can not bring down a plane flying at 31,000 feet and Isis would require a proper missile system. The Malaysian airlines, MH17 was shot down using a Buk missile at 33,000 feet.
Airbus A321 possibly destroyed by a bomb on board the plane: There is a growing speculation in the Russia media that the Metrojet flight 7K9268 disintegrated mid-air due to an on board explosion, probably a bomb.
According to Kommersant daily, a respected Russian newspaper noted that the way the Airbus A321 broke-up would happen only if a bomb exploded in the pressurised passenger cabin sending shock waves the broke the fuselage.
The view also has been supported by Professor Michael Clarke at the Royal United Services Institute think-tank, who told the BBC: "If we have to guess at this stage, it's much more likely to have been a bomb on board rather than a missile fired from the ground".
Technical issues caused the crash: Some experts have opined that since many victims were found with their seat belts on, there is a possibility that the plane might have experienced a technical issue, following which the pilot asked the passengers to put on the seats belts to avoid bumpy conditions.
Initially, an Egyptian aviation official also had claimed that the pilot had requested to make an emergency landing, before losing contact with the air traffic controllers.
Ranyah Sabry, a BBC correspondent in Cairo had reported that the plane was some 48 kms from the airport where it was heading to make an emergency landing when it crashed.
Aviation analyst Peter Goelz told CNN that it was "some sort of catastrophic failure, perhaps caused by an earlier maintenance problem. It could have been a center fuel tank that might have exploded."