The open-source prediction is that the plane was brought down by an on board bomb. As a prediction, it is almost too late to be interesting. The Brits say yes; Americans maybe; the Russians and Egyptians say, too early to tell.

But it’s not too late to examine the question for didactic purposes.  A shoulder fired missile, a.k.a. MANPAD,  cannot reach the cruising altitude of a commercial airliner.  Let’s fence it with facts which exclude mechanical failure:

• The airliner was an Airbus A321, of which there have been only two other hull losses, both due to pilot error. One hit a mountain in a bad-weather approach to Islamabad. Another hit a runway utility vehicle at Tainan airport Taiwan.
• The weather was fine, with none of the risk factors of Air France Flight 447, which was downed by a combination of pilot error, instrument malfunction,  and the notoriously violent thunderstorm supercells of the Intertropical Convergence Zone.
• Hence, the possibility of internal mechanical failure causing the breakup of the aircraft, while not excluded, is exceedingly small.

In open source analysis, the ratio of an outcome probability to alternatives is the relevant factor, not the absolute probability. In the case of TWA Flight 800, the chance of mechanical failure was also exceedingly small, but unlike the current case, vastly greater than competing theories, mostly  conspiratorial in nature.

The Muslim Brotherhood, which under Mohamed Morsi ruled Egypt between 2011 and 2013, has had many phases, or colors. With origins as a typical terrorist, or revolutionary organization, phases of moderation, or attempted political legitimacy, were punctuated by occasional violence, and many cycles of acceptance and repression, particularly under Anwar Sadat. Uncertainty about how coherently bound the Brotherhood was to a common platform at any particular time  was characteristic of external observers, and possibly of the Brotherhood itself.

What happens when you hit a splintery rock with a hammer, shattering it, sending political shards in all directions? Abdel Fattah el-Sisi hit the Muslim Brotherhood with a hammer, destroying whatever coherence bound Brotherhood members to a political ethic. Those who found themselves not in jail, and whose inner tendencies  were previously moderated by the pull of central Brotherhood leadership, became ISIS sympathizers.

This is not saying much about a country where an unwary foreign tourist at Cairo Airport may need  to be ransomed from a restroom, or buy a snatched passport back from a professional passport snatcher. So it is easy for ISIS to use disaffected members of the Brotherhood to reach inside Egyptian official institutions to place a bomb on an airplane. Apparently, Sharma-El-Sheikh Airport is a friendlier place, where for a small sum a traveler may buy a way around security, a kind of “frequent bomber program.”

So probabilities related to airplanes in general are minimal. An actor has been identified. Thus far, this is the easiest kind of open-source question to resolve, as close to proof as statistics can provide. Only the differing statements of the four “authorities” stand in the way of the conclusion:

• The Egyptians don’t want to admit the level of institutional compromise implied by the bomb diagnosis. For them, the best outcome, with the least impact on tourism, would be mechanical failure.
• The Russians, who already send 3M tourists per year to Egypt, would like even more comradely relations. Egypt was a Soviet client of longstanding; a resumption would be a major coup for those who dream of the Soviet reach of old. It’s amusing to think they may hang the Russian airline to get the conclusion they want.
• The Americans are just slightly piqued that the Brits found out first. To understand in fullness the special relationship, read every book by John le Carré.

Congratulations, Brits! You’ve done more with less.