New studies by the National Institutes of Health failed to find evidence of brain injury in scans or blood markers of the diplomats and spies who suffered symptoms of Havana syndrome, bolstering the conclusions of U.S. intelligence agencies about the strange health incidents.

Spy agencies have concluded that the debilitating symptoms associated with Havana syndrome, including dizziness and migraines, are not the work of a hostile foreign power. They have not identified a weapon or device that caused the injuries, and intelligence analysts now believe the symptoms are most likely explained by environmental factors, existing medical conditions or stress.

The lead scientist on one of the two new studies said that while the study was not designed to find a cause, the findings were consistent with those determinations.

The authors said the studies are at odds with findings from researchers at the University of Pennsylvania, who found differences in brain scans of people with Havana syndrome symptoms and a control group

Dr. David Relman, a prominent scientist who has had access to the classified files involving the cases and representatives of people suffering from Havana syndrome, said the new studies were flawed. Many brain injuries are difficult to detect with scans or blood markers, he said. He added that the findings do not dispute that an external force, like a directed energy device, could have injured the current and former government workers.

The studies were published in The Journal of the American Medical Association on Monday alongside an editorial by Dr. Relman that was critical of the findings.

The incidents began to occur in greater concentrations at the end of 2016 and in 2017 in Havana and later in China, Austria and elsewhere. The Biden administration took office in 2021 promising to improve health care for diplomats and spies suffering from the symptoms and vowing to get to the bottom of what was causing them.

Studies by the University of Pennsylvania in 2018 and 2019 suggested that people affected by the syndrome had possible brain injuries that were different from typical concussion injuries or other traumatic brain injuries.

The N.I.H. studies looked at a different group of people, with less than a third of the cases overlapping. Dr. Leighton Chan, the acting chief scientific officer for the N.I.H. Clinical Center and the lead author of one of the studies, said that of the 86 participants, 24 cases were from Cuba, six from China, 17 from Vienna, nine from around the United States and 30 from other locations.

While examining the brain scans, the researchers found no significant differences with the control group.

In a news conference discussing the results before their public release, the N.I.H. scientists said their scans, done in a research setting, were more precise than the scans produced primarily in clinical settings during earlier studies. They also said the control group was more closely matched to the study participants, improving the study’s rigor.

Researchers from the University of Pennsylvania said the two studies were “apples to oranges” comparisons because they looked at different groups of patients, and the N.I.H. study was not designed to replicate theirs.

The N.I.H. scientists said they did not diagnose the patients with traumatic brain injuries or concussions. The diagnoses they offered instead, all so-called “functional neurologic disorders,” are often caused by stress.

The studies did not rule out a potential external cause for Havana syndrome symptoms. But if one was not involved, Dr. Chan said, stress “may explain more of our findings.”

“It is important to note that individuals with functional neurological disorders of any cause have symptoms that are real, distressing and very difficult to treat,” Dr. Chan said.

The N.I.H. diagnosis angered several people with Havana syndrome symptoms who said it was insulting and misguided because it was tantamount to calling their symptoms psychosomatic or the result of mass hysteria.

Dr. Relman, who was among the leaders of an experts panel established by the intelligence agencies and another by the National Academy of Sciences, said the work of those groups had found that the symptoms of some of the affected government workers could not have been caused by stress or psychosocial factors alone.

The N.I.H. studies looked at a large group of people who reported diverse symptoms, rather than zeroing in on overseas cases where additional evidence shows something strange could have been going on, Dr. Relman said. In those cases, a concealable device, capable of delivering directed energy in a targeted way, could have been responsible.

“To lump all these cases together in the way they did is simply asking for trouble,” Dr. Relman said.

Mark Zaid, a lawyer for several people with Havana syndrome symptoms, said many current and former officials treated at N.I.H. were upset that they were not briefed on the study before it came out. Mr. Zaid said some patients were told that they had to participate in the study to receive treatment from the government for their symptoms. Mr. Zaid said that had raised ethical questions about the patients’ consent.

Dr. Chan disputed that and said that the people who participated did so willingly and could have left the study at any time.

But Mr. Zaid said he feared that the C.I.A. and other intelligence agencies would improperly use the study to bolster their findings that they could not determine an external cause for Havana syndrome cases.

“The concern is that intelligence community is going to weaponize this study to show that the absence of evidence is evidence,” Mr. Zaid said. “And it is not.”