A toxicology report showed he did have anti-anxiety medication in his system, based on a test of his urine. It was no longer detectable in his blood, meaning it was not active. The autopsy, performed by the Clark County coroner, and its related reports were obtained by The New York Times on Friday, and were first reported on by The Las Vegas Review-Journal, which had gone to court with The Associated Press to secure their release.

The examination showed that certain sections of Mr. Paddock’s brain contained small spheres that can be seen when the tissue is treated with a stain. The structures, made up mostly of carbohydrates but also containing proteins and other components, are known as corpora amylacea. They studded parts of his hippocampus — involved in learning and memory — and the frontal lobes, among other areas. “Most people would have them at that age, but not in that profusion,” Dr. Vogel said. “It’s a striking exaggeration of an age-related finding.”

The cause and meaning of the structures remain mysterious, the subject of ongoing research. They appear as people grow older and are found at greater density in the brains of patients with Alzheimer’s disease, multiple sclerosis and epilepsy, among other disorders. Dr. Vogel searched for signs of those diseases, but he did not detect them in Mr. Paddock’s brain.

Three scientists who study corpora amylacea said in interviews that the increased presence of the structures was significant. “If you have high numbers of these, something’s not normal,” said Troy Rohn, a professor of biology at Boise State University.

Other scientists believe the structures are akin to wastebaskets that contain remnants of broken-down cells or even infectious or hazardous substances.

Dr. Hyman Schipper, a professor of neurology at McGill University in Montreal, said his laboratory’s studies have found evidence that corpora amylacea can result from the damage caused by unstable molecules known as free radicals, which can be unleashed by various stressors. “It’s telling you something and it could be very important,” Dr. Schipper said of the finding in Mr. Paddock’s brain.

Even so, Dr. Vogel, the Stanford neuropathologist, said the results of his examination should reassure the public that Mr. Paddock’s doctors had not missed diagnosing a tumor or other major brain disorder that could have been treated.