Two of the young men had killed themselves. The scientists do not know if their head injuries had contributed to their suicides. The other two had died of brain swelling that most likely was related to “second-impact syndrome,” which can occur if someone experiences two head injuries within a short period of time.

“None of the individual impacts was serious enough, in and of itself, to have caused death,” says Dr. Lee Goldstein, an associate professor of psychiatry at Boston University School of Medicine and the study’s senior author.

But when the researchers closely examined the young men’s brains, they found more harm than they had expected. The teenagers’ blood-brain barriers, a natural defense system that keeps harmful substances from entering the brain, appeared to have been damaged, and many of the small blood vessels throughout their brains had sprung tiny leaks. Two of the brains showed disquieting accumulations of tau proteins near these broken blood vessels, and one brain had diagnosable Stage I C.T.E.

This was the first time that scientists had found signs of incipient or actual C.T.E. so soon after a brain injury and in people so young.

But since so many other factors might have contributed to the brain conditions, from genetics to earlier hits to the head, the researchers next decided to look at similar head impacts in animals and track precisely what happened inside their skulls.

Using young male mice, they applied relatively mild jolts, designed to result in a sudden, strong jerking of their heads, much as occurs during head-to-head tackles and other impacts. Afterward, some animals showed symptoms of a rodent version of a concussion, stumbling and performing poorly on memory tests.

The scientists then injected some animals with a dye that cannot cross a healthy blood-brain barrier and scanned the living animals’ brains. In about half of the mice, they saw signs of the dye in their brains, indicated that their blood-brain barriers had become permeable. Many of the mice also showed signs of leaky blood vessels and other damage, including inflammation and disruptions in the electrical activity within their brains. Some had early signs of tau accumulation.