Alicia Arritt spent years as an army nurse working with combat veterans with brain injuries. And when she started a relationship with Matthew Livelsberger in 2018, long before he shot himself and blew up a Cybertruck in Las Vegas this week, she recognised many of the symptoms in her new boyfriend that she had seen in her patients.

A master sergeant in the army’s 10th Special Forces Group, he was forgetting words, losing his train of thought midsentence and struggling with insomnia. He had headaches and depressive moods that sometimes kept him shut away for days. In a text exchange, after they started dating, he mentioned having been deployed three times in three years. She asked if he had been hurt. “Just some concussions,” he responded.

“I think he wanted to get help, but he thought if he said anything, he wouldn’t be able to do his job anymore,” she said in an interview on Friday from her home in Colorado Springs. They dated for two years and then remained friends.

By the time they met, Sergeant Livelsberger had been in the army for more than a decade and had been deployed into combat a number of times. He had spent years jumping from aeroplanes and being exposed to weapons blasts in training. He had back injuries from hard parachute landings and had lost some of his hearing from being around explosions and gunfire.

The military has begun to recognise in recent years that routine operations can, over time, cause brain injuries, and Congress has passed legislation requiring the military to better track blast exposure and provide treatment. But in combat units, many troops still don’t report injuries for fear of being put on the sidelines.

The army, in a statement after this article was published, said that Sergeant Livelsberger had access to programmes provided for mental health and “did not display any concerning behaviours at the time”.

Despite his troubles, Sergeant Livelsberger was also kind, funny and intelligent, Arritt said. He liked to hike, camp and play with her dogs. She said there was nothing in his talk or in his actions that suggested he was inclined to carry out a violent act like the one on Wednesday, when, according to the police, he detonated explosives in front of the Trump International Hotel.

“He was just a really loving guy with a deep well of integrity,” she said.

At a news conference on Friday, the head of the FBI’s Las Vegas field office said that Sergeant Livelsberger, who was 37, was a “heavily decorated combat veteran who was struggling with PTSD and other issues”.

Investigators are still gathering and examining records and did not provide any specific information about a diagnosis of post-traumatic stress disorder or other mental health or neurological conditions.

To Arritt, Sergeant Livelsberger had symptoms of a traumatic brain injury, but he didn’t have a diagnosis from the army. Over the years, she said, his symptoms seemed to grow worse.

Repeated blows to the head and blast exposure may cause damage that can accumulate almost unnoticed over time, said Dr. Michael Jaffee, a retired Air Force colonel and a neurologist who was director of the military’s Defense and Veterans Brain Injury Center and now runs a similar centre at the University of Florida. Neurologists now measure the risk of developing a brain injury, including the progressive disease chronic traumatic encephalopathy, or CTE, not in terms of the number of concussions sustained, but in terms of time involved in high-risk activities.

“The more years of exposure, the higher the risk,” Dr. Jaffee said. One of the high-risk activities officially listed by the National Institutes of Health, he noted, is “military service”.

It is impossible to definitively diagnose in living patients the type of progressive brain injury that can result from repeated, low-level exposure; it can only be diagnosed through post-mortem in an autopsy. Because of this, Dr. Jaffee said, the problem is routinely misdiagnosed or missed entirely. “There’s a reason it’s called an invisible injury. It’s not always obvious.”

The medical examiner’s office for Las Vegas did not respond to a request from The New York Times regarding whether it planned to test Sergeant Livelsberger’s brain.

Arritt said she did not think the sergeant had ever been formally diagnosed with a brain injury. Even so, a person like Sergeant Livelsberger, with nearly 20 years in special operations and a history of contact sports, would have been at high risk.

In an interview, Dr. Jaffee listed the symptoms associated with CTE. Many were identical to what Arritt said she had seen in Sergeant Livelsberger.

In some cases, Dr. Jaffee said, the symptoms of brain injury can plateau. In others, he said “the disease is degenerative — there is a downward progressive course”.

Some service members who have high exposure often compensate for years then suddenly fall apart, often around age 40. A few begin to exhibit bizarre behaviours, including paranoia and delusions. It has happened with Navy SEALs late in their careers, with the elite boat crews as they reach senior ranks after years of wave-crashing missions and with TOPGUN fighter pilots repeatedly exposed to high G-forces.

A small number, including the army reservist Robert R. Card II, who killed 18 people and himself in Maine in 2023, turn violent. An autopsy found that Card’s brain had extensive damage consistent with blast exposure. An army investigation found no link between Card’s mental health problems and the years he spent working as a grenade instructor, repeatedly exposed to explosions on the practice range.

“We can never say that having repeated head injuries will make someone do something violent,” said Dr. Daniel Daneshvar, the chief of the Division of Brain Injury Rehabilitation at Massachusetts General Hospital. “But we can say that repeated head injury increases your probability of making very bad decisions.”

New York Times News Service