Perl and his lab colleagues recognized that the injury that they were looking at was nothing like concussion. The hallmark of C.T.E. is an abnormal protein called tau, which builds up, usually over years, throughout the cerebral cortex but especially in the temporal lobes, visible across the stained tissue like brown mold. What they found in these traumatic-brain-injury cases was totally different: a dustlike scarring, often at the border between gray matter (where synapses reside) and the white matter that interconnects it. Over the following months, Perl and his team examined several more brains of service members who died well after their blast exposure, including a highly decorated Special Operations Forces soldier who committed suicide. All of them had the same pattern of scarring in the same places, which appeared to correspond to the brain’s centers for sleep, cognition and other classic brain-injury trouble spots.

Then came an even more surprising discovery. They examined the brains of two veterans who died just days after their blast exposure and found embryonic versions of the same injury, in the same areas, and the development of the injuries seemed to match the time elapsed since the blast event. Perl and his team then compared the damaged brains with those of people who suffered ordinary concussions and others who had drug addictions (which can also cause visible brain changes) and a final group with no injuries at all. No one in these post-mortem control groups had the brown-dust pattern.

Perl’s findings, published in the scientific journal The Lancet Neurology, may represent the key to a medical mystery first glimpsed a century ago in the trenches of World War I. It was first known as shell shock, then combat fatigue and finally PTSD, and in each case, it was almost universally understood as a psychic rather than a physical affliction. Only in the past decade or so did an elite group of neurologists, physicists and senior officers begin pushing back at a military leadership that had long told recruits with these wounds to “deal with it,” fed them pills and sent them back into battle.