Hoofprints of horses, not zebras.
An adage that many doctors repeat, having learned it in medical school: "When you see hoofprints, think horses, not zebras." Most of the time, the symptoms, though one may want to see a rare and exotic ailment, are of much more prosaic maladies.
You are seeing zebras. Maybe even unicorns.
It's perfectly plausible, and even likely, that terrorists committed an atrocity and bragged about it. They do this all the time. They have no shame. It's part of who they are.
Although outrage and sorrow are the natural responses to such an act, it has no bearing on our responsibility to conduct ourselves as a civilized nation.