Alicia Arritt spent years as an Army nurse working with combat veterans with brain injuries. And when she started a relationship Matthew Livelsberger in 2018, long before he shot himself and blew up a Cybertruck in Las Vegas this week, she recognized 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 more than a decade and had been deployed into combat a number of times. He had spent years jumping from airplanes 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 recognize 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.

Despite his troubles, Sergeant Livelsberger was also kind, funny and intelligent, Ms. 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.

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