It is appearing likely that it was a BOMB that brought down the Russian MetroJet full of Russian tourists, although there still is the possibility that a poor repair from a tail strike in 2001 could have led to a structure failure that led to instant decompression. That could also sound like a bomb explosion.
However, there are reports that ISIS traffic the day before the explosion hinted of "something big" about to happen the day before the crash.
Think about it. As it spreads into these weak areas of little control, where airport security is very lax and slipping a suitcase stuffed with explosives into a cargo hold would be very easy. Who would notice an extra bag?
Hitting a Russian jet would make a point -- what country just starting a bombing campaign in Syria, land of ISIS/ISIL? This was a message to Putin, who like GW Bush, just discovered that military action makes your own citizens a target. As a result, 80,000 Russian are now stranded in Sharm el-Sheik, waiting flights home.
After all, it was the American boots on the ground in Saudi Arabia during the first Gulf War against Saddam is what angered Osama bin laden into developing the 9/11 attack on the U.S. homeland. He saw "infidels" on sacred territory (Saudi is home to the birth of Islam).
ISIS can do what Osama did, but on a smaller scale, simply by targeting aircraft in less secure airports surrounding its territory in Syria/ Iraq. Just look at this map:
Putin has inherited ISIS wrath as a result of his bravado and acts in Syria. Now his citizens are paying a price for his ego and actions, which are always praised and never questioned by Russian media.
All airlines and passengers are now at risk of this potential new threat. For years, it wasn't the passengers boarding that have posed a problem, other than misbehaving after indulging in too much alcohol during a flight, because they had to nearly strip to get past security and on the plane. But the cargo being placed below them doesn't get the same scrutiny or security scans. That's the weakest link of the system, and lax security at places like Sharm el-sheikh.
A pressure bomb is a perfect weapon - simple, easy to use and inconspicuous when packaged like everything around it.
On top of that another airliner reported that a MISSILE came within 1 mile of it during a recent flight! Egyptian military could have fired it, which doesn't make one feel confident. MadPads can shoot down an airliner up to 11,000 feet, like during a landing or takeoff...
Stay tuned for updates.