The investigators will also be verifying who worked at the airport that day and who was able to gain access to the aircraft, including ground staff, pax, crew, an aviation inspector perhaps?
Good luck with that part of the investigation, pal. After having been stranded as a pax in Cairo airport overnight, and watching the lip-service paid to security between landside and airside, I can assure you, I never want to transit Egypt again, nor be part of their air travel system.
Many people seem to assume that terrorist organisations are some kind of highly organised groups with tight control over their members. The exact opposite is true.
These people are criminally-inclined sociopath nutters who regularly break up into small splinter groups, and who will fight each other as bitterly as they will fight European or Christian countries.
It's becoming more and more likely - despite the inadequate amount of evidence being released - to assume that a bomb brought down this aircraft and that said bomb was easily placed in the rear of the cargo hold by a terrorist from a small splinter group originating from Syria - which individual was intent on venting their hatred on a Russian civilian aircraft and Russian civilians, due to Russian warplane activity in his family or fighter friends environment.
No need for them to claim responsibility or make a big noise about it - they have carried out their revenge, and any attempt to gain pyschological advantage by advertising that they were responsible, would only bring intense and immediate electronic, military and investigative scrutiny upon them. Far better to operate like a cockroach and avoid the limelight. And human cockroaches describes ME terrorists particularly well.
The fact that a large portion of the fuselage between the wings and the tail is no longer intact - as one would expect in the case of a HS or pressure bulkhead failure, leads to the inevitable conclusion that only a bomb could be responsible.
The A321 is a basically-sound construction aircraft, with no important known weaknesses - and pressure bulkhead failures are a particularly rare event - but they have assumed a greater fear than deserved amongst many people, who tend to place a far greater chance of this event happening, than is likely to occur.