There are broadly four areas investigators will consider as they probe the disappearance of EgyptAir Flight 804 over the Mediterranean shortly before it began its descent to Cairo: Did it crash or explode due to pilot error, a technical malfunction, a smuggled bomb or an inside act of sabotage?
The hunger to know the cause of any plane crash is immediate and understandable -- both for the families of those on board and for any broader safety implications.
Ahmed Adel, vice chairman of EgyptAir, told CNN's Christiane Amanpour that in this case "it could be mechanical failure; it could be a terrorist attack."
"There are so many reasons that anything could happen up in the air," he said.
Bad weather does not seem a viable scenario. It was a fine night over the eastern Mediterranean. The aircraft was at its cruising altitude and the last conversation the crew had with Greek controllers did not include any discussion of weather conditions.
Scenario 1: Smuggled bomb
An act of terrorism is uppermost in the minds of some officials. Egyptian Aviation Minister Sherif Fathy suggested terrorism was more likely than technical failure.
"If you analyze this situation properly, the possibility of having a different action aboard, of having a terror attack, is higher than having a technical problem," he said in Cairo.
The early theory among U.S. officials also is terrorism, with the initial suspicion that the plane was taken down by a bomb, they have told CNN. That theory, they caution, is based on the circumstances, not concrete evidence. Nor was there any known threat to the flight, the officials said.
If, and it remains a substantial if, EgyptAir Flight 804 was brought down by a bomb, it would be another stark reminder that security measures at airports are only as good as the people implementing them.
In an article for the new edition of the Combating Terrorism Center's Sentinel, Robert Liscouski and William McGann write that "terrorist groups from Yemen to Syria to East Africa continue to explore innovative ways to get bombs onto passenger jets by trying to beat detection systems or recruit insiders."
They say that "layered state-of-the-art detection systems that are now in place at most airports in the developed world make it very hard for terrorists to sneak bomb onto planes." These include high-definition or multiview X-ray machines and systems that involve a screener taking a swab of a surface of an object or your clothing and testing it in an explosives-trace detection machine.
But, they added, "Many airports in the developing world either have not deployed these technologies or have not provided rigorous training for operators."
Liscouski and McGann cite the incident aboard a flight that left Mogadishu in Somalia on February 2. Two airport workers were able to get a laptop containing an explosive device through an X-ray checkpoint. They handed it to a man who was due to fly out on a Turkish Airlines flight. When that was canceled, he left on another flight for Djibouti.
Twenty minutes into the flight, the laptop exploded, blowing a large hole in the fuselage. The man carrying the laptop was sucked out of the plane.
"Only the fact that the plane had yet to reach cruising altitude.... likely saved the lives of the more than 70 passengers on board the plane," which made an emergency landing, wrote Liscouski and McGann.
That attack was claimed by al Qaeda's Somali affiliate, al-Shabaab.
Liscouski and McGann bring decades of experience to their study. Liscouski was assistant secretary for infrastructure protection at the U.S. Department of Homeland Security. McGann helped develop spectrometry technology for detecting explosive traces. The Combating Terrorism Center is associated with West Point.
Scenario 2: Insider threat
The Mogadishu incident again highlighted the "insider threat" to aviation security. In November last year, an airport employee allegedly helped smuggle a bomb on board a Russian Metrojet airliner at Sharm el-Sheikh airport in Egypt, though Egyptian authorities say no one has been arrested or charged in connection with the attack.
That bomb -- placed on an Airbus A321, exploded soon after the plane reached its cruising altitude; all 224 people on board were killed. Within hours, the ISIS affiliate in Egypt claimed responsibility. Later, the online ISIS magazine Dabiq included a photo of a soda can that it claimed was an improvised bomb placed on board the flight.
Liscouski and McGann say that many airports in the developing world lag in deploying state-of-the-art machines and rigorous training. Additionally, terrorist groups can recruit airport insiders "who either are likely to receive less scrutiny from fellow airport staff at security checkpoints than passengers or can evade screening altogether."
U.S. Rep. Michael McCaul, chairman of the House Committee on Homeland Security, said Thursday "that you can have the best technology to screen, but if you have an insider threat that's somebody corrupted or radicalized or bribed, that's scary."
U.S. officials have told CNN that in the EgyptAir investigation, there will be immediate focus on the ground crew and anyone who had access to the plane in Paris -- as well as the aircraft's flight crew.
There has been a history of radicalization among airport workers at Charles de Gaulle Airport.
A French Interior Ministry document in 2004 said there were "illegal prayer sites at the airport used by several Muslim airport workers" who at the time "belonged to mosques preaching radical Islam."
Some of these workers had access to what is known as "airside" -- the aprons beyond security checkpoints -- and were "known by intelligence services as radicalized individuals and who are under a fiche S" -- a designation by French law enforcement to flag someone who could be a security threat.
The document said it was "particularly difficult to monitor every single temporary airport worker."
French authorities have taken multiple measures to enhance screening of employees and passengers since that document was written; and there is no evidence that a bomb or weapon has ever been smuggled onto a plane at Charles de Gaulle.
But bombs have made their way onto planes elsewhere.
Al Qaeda in the Arabian Peninsula (AQAP) managed to get two printer bombs onto planes at Sanaa airport in Yemen in October 2010.
Its master bomb-maker, Ibrahim al Asiri, designed devices that concealed 400 grams of the high-explosive PETN inside printer cartridges and were timed to go off mid-flight on U.S.-bound aircraft. The devices were found as a result of an intelligence tip, not because of security screening. By then they had arrived in the United Kingdom and Dubai respectively -- destined for Chicago.
Similarly, AQAP operative Umar Farouk Abdulmutallab came close to detonating a device that he'd prepared inside a lavatory on a flight from Amsterdam to Detroit on Christmas Day 2009. The device failed to explode, but experts said it was a viable design.
If evidence of sabotage is substantiated in the case of EgyptAir 804, the immediate questions will be where, when and how was a bomb put on the plane?
In the previous 24 hours, the plane had traveled from Cairo to Asmara, Eritrea, and returned. It made a round trip to Tunis. It then left Cairo for Paris in the late afternoon. It left for Egypt late Wednesday.
Experts caution that the longer a device is on board a plane, the more likely it will be discovered, and the more difficult it would be for terrorists to predict whether the plane would be on the ground or in mid-air when the device explodes. But timing devices for a long delay are feasible -- as the Asiri-made printer-bombs showed. The Irish Republican Army planted a bomb at a hotel in Brighton, England, in September 1984; it exploded nearly a month later.
Scenario 3: Technical problem
Other scenarios, or a combination of pilot error and a technical problem, are equally plausible in the EgyptAir investigation. Much will hinge on how quickly wreckage is recovered, and which parts of the plane can be examined.
The part of the Mediterranean where the EgyptAir flight vanished is closer to land and much less deep than -- for example -- the mid-Atlantic, where Air France Flight 447 crashed in June 2009 while en route from Rio de Janeiro to Paris. The flight data recorder and cockpit voice recorder on that plane were eventually found at a depth of nearly 4,000 meters (more than 2 miles) -- nearly two years later.
The area of the Mediterranean beneath the EgyptAir flight's last confirmed position is in most parts some 2,000 meters deep -- still a daunting task, depending on how widespread the debris field might be.
The Air France crash was ultimately blamed on a combination of faulty data reaching the pilots -- because pitot tubes, or pressure measure instruments, had frozen and provided the wrong air speed -- and the muddled response from the two pilots who were at the controls at the time.
The French accident investigation report concluded that "the failure of the attempts to understand the situation and the destructuring of crew cooperation fed on each other until the total loss of cognitive control of the situation."
The highly experienced captain was on a rest break at the time of the emergency.
The reasons for the disappearance of Malaysia Airlines Flight 370 in 2013 are still unknown because the crash site was never accurately located and exhaustive searches have so far yielded no results.
While some small parts of the plane have washed up on coastlines thousands of miles to the west, no critical part of the plane has been recovered. Nor was there any communication from the plane for hours as it flew south from its intended flight path.
The many advances in aircraft technology have made modern planes much less prone to mechanical or other technical failure. "There were no snags reported in the technical log," of the Airbus, according to EgyptAir's Adel.
But maintenance is key. In 2014, an Air Asia flight that left Surabaya in Indonesia experienced repeated failures in its rudder travel limiter units because of a cracked solder joint.
Terrorism was at first suspected in the sudden explosion of TWA Flight 800 soon after it left New York in 1996. The U.S. National Transportation Safety Board found the probable cause of the accident was "an explosion of the center-wing fuel tank resulting from ignition of the flammable fuel/air mixture in the tank" most likely caused by a short circuit.
Scenario 4: The pilots
EgyptAir says the captain of Flight 804 had 6,275 flying hours, including 2,101 on the A320; the co-pilot had 2,766. They were both experienced.
According to aviation analysts, the high level of automation aboard modern planes reduces the scope for catastrophic error among pilots. But when something does go wrong, the flight crew are bombarded with information and, in some cases, have difficulty coping with it.
One example was the crash-landing of an Asiana flight approaching San Francisco airport in 2013, in which three passengers were killed. The NTSB report said the junior pilot at the controls failed to grasp "the complexities of the auto-throttle and autopilot flight director systems."
There have also been two occasions in recent years when pilots themselves have been blamed for intentionally crashing their planes -- in each instance when their colleague was out of the cockpit.
The Germanwings crash in March last year was caused by the 27-year old co-pilot, Andreas Lubitz, who locked the captain out of the cockpit before setting the plane on a planned descent into mountains in the south of France.
And in 1999, an EgyptAir Boeing 767 plunged 14,000 feet in 36 seconds soon after leaving New York. Again, initial suspicion was that a bomb had brought down the plane. The NTSB found that the plane crashed as a "result of the relief first officer's flight control inputs," but could not conclude why he had taken such action.
Past investigations offer many reasons to be cautious in analyzing just why EgyptAir Flight 804 suddenly disappeared in the early hours of Thursday morning.
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