Theories of what caused the tragedy have changed dramatically over a week, but this much we know
So, was it a bomb?
A flight accident investigator has told France 2, a French TV station, that an explosion can be heard on Metrojet Flight 9268's flight recorder, adding to the growing catalogue of evidence pointing to the Russian plane having been brought down by a bomb.
Furthermore, it has been reported that a US satellite picked up a heat flash before the aircraft went down, killing all 224 passengers and crew.
The key evidence pointing towards a bomb could turn out to be a blast hole with tightly curled edges, shrapnel on seat cushions or on the bodies, metal circuits that melted, or swabs of the metal tested for traces of explosive.
If so, who planted the bomb?
Communications intercepted by US intelligence agencies are said to show Isil leaders in their stronghold in Raqqa in Syria, "boasting" of bringing down a plane and even how they did it.
The communications, reported yesterday, are said to be with a jihadist group based in the Sinai Peninsula, where Flight 9268 crashed last Saturday.
"They were clearly celebrating," a US official told NBC News.
The "chatter" is also said to include a previous message from the Sinai group warning of "something big in the area" before the crash.
British government sources have also reported "chatter" pointing to a bomb, which intelligence agencies picked up through their listening base at RAF Troodos, in Cyprus, one of the most important of the overseas stations run by Britain's surveillance agency GCHQ.
The Sinai group, which goes under a number of guises including Ansar Beit al-Maqdis (ABM), Sinai Province or Supporters of Jerusalem, claimed responsibility in a four-minute statement circulating on social media a day after the disaster.
Then four days later, the group released an audio tape, purportedly read out by its spokesman Abu Osama al-Masri, claiming: "We are the ones who brought it down."
The Sinai jihadi group ABM pledged allegiance to Isil a year ago in a nine-minute audio speech released online. Last week, it claimed to have bombed the Russian jet 20 minutes into its journey from Sharm el-Sheikh to St Petersburg in celebration of the first anniversary of its affiliation to IS.
ABM was formed in 2011, declaring itself al-Qaeda's wing in Sinai, and it began attacking Israeli targets.
By 2014, it had become Egypt's "most dangerous militant group", led by Hisham al-Ashmawy, a former special forces officer in the Egyptian Army turned jihadist.
"Ashmawy is the mastermind and executor," an Egyptian security official told Reuters last month.
Other sources suggest that when ABM pledged loyalty to IS, Ashmawy stuck with al-Qaeda and fled to a training camp in Libya with a death sentence on his head.
The Sinai group was initially made up of Bedouin but has attracted a growing number of 'mainland' Egyptians as well as members from Sudan and Yemen; many went to Syria for training..
The possible bomb attack is being seen by security analysts as a 'game changer' for IS: "For the first time we are seeing the full attention of the Islamic State when it wants to wreak havoc," said Mokhtar Awad, an analyst at the Centre for American Progress who focuses on Islamist extremism.
"We're seeing what happens when they dedicate their resources to actually deploy terrorism as a tool of retaliation against Western and Russian airstrikes," he said.
How was it carried out?
In claiming responsibility, Abu Osama al-Masri said the method for downing the jet would be revealed "in the time of our choosing".
So far, the terror group has declined to go beyond that but the prime theory of intelligence agencies is an explosive was placed in the luggage hold just prior to take-off.
A bomb may have been already placed inside a suitcase which was then loaded on to the plane or else one was simply stuffed among the luggage by a baggage handler. The decision by David Cameron to suspend flights to Sharm el-Sheikh was based partly on a review of CCTV footage of the airport's baggage handling systems by British aviation officials.
Experts from the Department of Transport looked at video of how the system was run. What they saw led them to recommend grounding flights on Wednesday night.
It is understood that analysis of "chatter" is one of the things which allows British security sources to say a bomb could have been placed on board by a member of the airport staff motivated by money or ideology.
Whitehall sources have told The Telegraph one theory was the bomb may have been smuggled on board among the hot meals served to passengers.
Britain is demanding detailed answers from the Egyptians about their baggage handling staff, how they are screened before being hired and what security checks they face when arriving for work.
Egyptian investigators have also questioned cleaning and catering staff at the airport.
Ministers warned last month of the continuing threat of liquid bombs, which can evade detection in metal detectors and x-ray machines, and even a cyber attack in which an aircraft could be controlled and then downed by a computer hacker on the ground.
Did governments miss the warnings?
Just 18 days ago, Lord Ahmad of Wimbledon, the aviation and transport security minister, warned that one of the biggest dangers to civilian flights was plotters beating security measures at overseas airports.
He said attempts to smuggle liquid explosives on board remained a danger, as did other devices designed "to evade detection" by airport security.
In a speech to industry bodies he said: "My biggest concern on security is not to do with technology.
"It is to do with human behaviour. It's important screeners at overseas airports do a good job too."
His warning is compounded by the realisation that Islamist insurgents in Egypt had presented a growing threat to tourists, particularly around the Sinai peninsula.
Three South Korean tourists and their driver were murdered by ABM, the group that has claimed responsibility for blowing up the Russian jet, in February last year during an attack on a bus in Taba, Sinai.
Then just over a month ago a terror threat led to the cancellation of Egypt Air flight M5 76 from Brussels. On August 23, just over two months before the Russian Airbus came down, a British passenger jet approaching Sharm el-Sheikh was forced to take evasive action after crew spotted a missile in its vicinity.
The Thomson flight, carrying 189 passengers, managed to land safely after the first officer in charge turned the plane to the left to avoid the rocket.
Thomson bosses immediately reported the incident to Department for Transport staff who investigated with other Government agencies.
But the DfT concluded "there was no cause for concern" and that it was safe to continue flying to Sharm el-Sheikh.
A DfT spokesman said yesterday: "We investigated the reported incident at the time and concluded that it was not a targeted attack and was likely to be connected to routine exercises being conducted by the Egyptian military in the area at the time."
Foreign Office advice about travelling to the resort remained unchanged until the Russian plane crashed.
Until then the FCO had advised British citizens against all but essential travel to south Sinai, except to Sharm el-Sheikh, where "enhanced security measures are in place" to safeguard visitors to the resort.
But questions remain as to why details of the near-miss were not made public at the time, allowing travellers to Sharm el-Sheikh to decide whether to continue flying.
Sir Gerald Howarth, chairman of the all-party parliamentary group on Egypt, said: "It seems odd and certainly exceptional to have a missile being fired near a commercial aircraft.
"The British authorities need to explain what they know about this incident, whether there was a real threat and whether that threat could be repeated."
How are other countries reacting?
After initially being adamant that its plane had not been brought down by a bomb, Russia has now halted all flights to Sharm el-Sheikh and is evacuating its citizens from there. Russian airlines are sending empty planes to Egypt to bring home some of the estimated 80,000 Russians in the Red Sea resort.
More than 40 flights were expected to have left Sharm el-Sheikh on their return to Russia yesterday, with another 47 planned today.
Denmark, Norway and Finland yesterday joined countries already telling their citizens to avoid non-essential travel to Sharm el-Sheikh.
Norway's Foreign Ministry said it was also urging people already in the resort not to travel further around the Sinai Peninsula, because of the danger from continued fighting between Egyptian government forces and Islamic insurgents.
Finland made a similar recommendation to its citizens.
At the same time the US Department of Homeland Security announced that it was enforcing extra security measures on flights to the United States from around ten airports across the Middle East region, including Cairo, Kuwait International and Amman, in Jordan. Jeh Johnson, DHS secretary, said: "While there are no direct commercial air flights from Sharm el-Sheikh, Egypt to the United States, these enhancements are designed to provide an additional layer of security for the travelling public."
What is Egypt saying?
Egypt has reacted angrily to Britain's response to the destruction of the Russian aircraft. After initially denying suggestions the flight had been blown up by terrorists and criticising Britain's decision to pull its citizens out of Sharm el-Sheikh, the Egyptian government has now accused other countries of not sharing the intelligence information they have gathered on the crash.
Sameh Shoukry, the country's foreign minister, also criticised unnamed countries for failing to co-operate in fighting terrorism, accusing them of "concentrating on their self-interests" and underestimating the threats facing his country.
John Casson, the UK ambassador to Egypt, said he had been in close personal contact with the Egyptian government around the clock.
"We've made all the information available and we explained to them the basis for our decision-making," he told the BBC.