The determination of what brought down Metrojet 7K-9268 has not yet been made, but everyone has seen the leaks coming from western intelligence pointing to a bomb. So let's assume for a moment it was a bomb.
And let's assume it was planted by an operator on behalf of ISIS, due to the Russian connection.
Who might be the mastermind? We know AQ has some good bombmakers. AQAP's al-Asiri is notorious and a highly sought after drone target. We know of bombmakers in the Khorasan Group in Syria (otherwise known as Core AQ). One was reportedly just killed. But we haven't heard of many bombmaker geniuses populating the ranks of ISIS.
this guy? He's a little old, yes, but passing knowledge would be his main contribution. He's a long-time resident of Iraq, still wanted years after creating a bomb that was placed on a Pan Am 747 flying to Honolulu in 1982. At the time he was also working on a plot to bomb multiple aircraft before the plot was fortunately foiled. This was years before the Yousef/KSM "Bojinka" plot was uncovered. The Saddam government likely knew about this activity and looked the other way meaning they sanctioned it, for what it's worth.
Ironically, ISIS is filled with ex-Saddamists. Al-Umari is a Sunni loyalist, no doubt. Many of the ex-Saddamists have taught their jihadist partners a few tricks of their former terror trade, which has resulted in a new, improved terrorist army featuring many of the same enemies Saddam had. The only difference is the new group wears religious hats and dons beards.
So it sounds reasonable to assume he might be working with them now if he's still active in the game. At any rate, the United States has put up 5 million bucks in an effort to get this old man off the Arab streets, so they must have a good reason.
One thing to note. It's a bit strange how quickly and adamantly that "unnamed US security officials" have declared this a bomb. Here's yet another story. Even in the story the officials claim they are not directly involved in the investigation. Others say they have terrorist chatter in the NSA intercepts, but that's something they will never release due to security. Compare all that to the other mysterious aircraft crashes over the past 20 years, wherein officials went out of their way to deny terrorism was even possible. Obviously a political component needs to be suspected. This still could have been a mechanical issue.