Christian Beyer worked around the ground-shaking blasts of one of the Army’s most powerful weapons — the M1 Abrams tank — for 23 years. And for nearly all that time, he was a model soldier, given awards for meritorious service and promoted all the way up to master sergeant in charge of training young tank crews.

Then in 2020, at age 38, he started to fall apart.

He couldn’t sleep. His family noticed that his balance had turned unsteady and he began to slur his speech. He would weep about small things and dwell on imagined conspiracies.

He grew mean, then dangerous. One night late in 2021, according to Army documents, he shoved his wife during an argument and then grabbed for a kitchen knife when a senior sergeant tried to calm him down.

As Sergeant Beyer’s struggles were beginning, the military was just starting to recognize that firing heavy weapons could lead to brain damage. Under a mandate from Congress, it began to set up programs to track and limit exposure, announced its first safety threshold for blast intensity to avoid “adverse brain health outcomes,” and drew up a list of 14 weapons that might pose a significant risk.

But the M1 Abrams was not on the list. Tests of the tank’s 4,000-pound main gun found it to be well below the new safety threshold. So the Army continued to treat it as safe, and Sergeant Breyer kept on working.

The threshold is less scientific than it appears, though. It is not a measure of risk to brain health based on rigorous research, but rather a decades-old guideline for which blasts are likely to burst soldiers’ ear drums, borrowed in a pinch because the Pentagon had nothing better. Blast researchers say it may not offer a reliable gauge.

Christian Beyer joined the Army in 2000 when he was 17 and spent the next 23 years around tanks.Credit…Cece Dahl

“It’s basically a place holder, because no one knows what the real number should be,” said Christian Franck, a professor of biomechanics at the University of Wisconsin-Madison who is part of a team that is modeling the effects of blasts on the brain for the Defense Department. He echoed the assessment of many other researchers.

“If the right kind of wave hits brain tissue, the tissue just breaks — it literally gets torn apart,” Dr. Franck said. “We see that in the lab. But what kind of blast will do that in real life? It’s complex. The work takes time. There is a lot we don’t know.”

A vast majority of blasts happen in training, not combat, and instructors are often exposed to far more than anyone else. But unlike troops on combat deployments, instructors are not screened for brain injuries.

As Sergeant Beyer’s condition worsened, his wife, Christy Beyer, said she started to worry that he had been exposed to too many tank blasts, and told his command that he needed a thorough medical evaluation.

The Army decided he needed something else: punishment.

Instead of sending him to one of the military’s specialized brain injury centers to see whether a brain injury or some other factor might have caused his deterioration, commanders sent him to a court-martial for pushing his wife and other crimes related to the incident. It’s still unclear whether his change in behavior stemmed from a brain injury. The Army jailed him, reduced his rank, and then this spring forced him to retire.

“Something had changed in his brain,” Christy Beyer said recently in an emotional interview, often stopping to cry. “I kept begging people, his command, the police to get him help, but no one did.”

Out of uniform, Mr. Beyer continued to spiral out of control. He left his wife and three children and wandered the country with little direction. In October, the police say, he pulled a knife on two gray-haired men in Northern California during a dispute over parking, tried to run them over with his car and then fled, scrambling through vineyards, fording a river and setting off a multiday manhunt.

Mr. Beyer was arrested in Petaluma, Calif., in October.

The decorated former master sergeant is now in jail, awaiting trial on state and federal criminal charges. He has pleaded not guilty. Mr. Beyer could not be reached for comment, and his lawyer declined to comment.

The Defense Department’s new blast safety threshold measures the power of explosions based on the blast wave’s peak pressure: Anything more than four pounds per square inch is considered potentially harmful.

But peak pressure is just one measurement and blast waves are complicated, according to Dr. Franck, the blast researcher. Some waves reach a high peak but pass quickly, while others with lower peaks may last longer and deliver more energy.

While much about how blasts injure brains remains unknown, the military has been studying their effect on lungs and ears for decades, and in those studies some waves with lower peaks “can actually have more insult to the human body,” said Timothy Kluchinsky, an Army researcher who studies weapons safety.

He agreed that the current safety threshold might not be the best way to assess blast hazards. But he said the department needed to establish some sort of metric to warn the armed services to treat weapons blasts with caution.

The existing eardrum threshold was, Mr. Kluchinsky said, “the easiest thing for the user community to understand.”

Nearly all the weapons on the Defense Department’s list of 14 potentially hazardous weapons have measured below the safety threshold. But troops firing them have repeatedly reported concussion-like symptoms.

The military has begun an effort called the Warfighter Brain Health Initiative to coordinate research, safety guidelines and the tracking of blast exposure, but it is still a work in progress, rolled out quickly before it was complete, according to Kathy Lee, the director of the initiative.

“We need to be providing solutions,” Ms. Lee said in an interview. “If we can reach the 80-percent solution and get it out there, it may not be it perfect, but we can evolve as we need to.”

She said the safety threshold was part of a comprehensive new approach, including a program that would regularly test all troops for brain injuries starting next year.

Few people would dispute that the M1 Abrams tank delivers a punishing blast. Troops are required to wear double ear protection if they are within 800 feet, and must stay at least 30 feet behind the tank when it fires.

The tank is designed to be sealed during firing to protect the crew inside, but in practice crews regularly keep the top hatches open. That allows the blast to echo through their compartment, where the Army says it peaks at about two pounds per square inch. The boom they hear has been measured at 172 decibels — more than 100 times the sound intensity of a jet engine.

“It’s quite violent,” said Daniel Gade, a retired Army lieutenant colonel who commanded a tank company in Iraq. “I don’t think there is a guy who has fired it that doesn’t think it takes a toll.”

An Army spokesman, Lt. Col. Robert Lodewick, said all available data suggested that the risk to tank crews from blast exposure was “low.” In more than 20 years, he said, the Army has not recorded any brain injuries related to firing the Abrams gun.

Blast research is evolving, Colonel Lodewick said, adding that “the Army remains committed to understanding, mitigating, accurately diagnosing and promptly treating blast overpressure and its effects in all forms.”

That can be difficult, though, because the symptoms of a brain injury can take years to develop, and may include depression, anger, mood swings and other problems that are often mistaken for unrelated mental health conditions.

Artillery soldiers who were exposed to thousands of M777 howitzer blasts and were eventually diagnosed with brain injuries told The New York Times they were often initially misdiagnosed with psychiatric problems or punished for misconduct.

Mr. Beyer joined the Army right after high school and had an unusually long career spent entirely around tanks, including six overseas deployments, three of them to Iraq. His final assignment was as a tank instructor at one of the Army’s busiest training posts, Fort Irwin in California.

A typical tank crewman fires about 120 rounds a year, and works around tanks only for a few years before leaving the military or moving on to other assignments. Mr. Beyer never moved on.

A spokesman for Fort Irwin said that while most training was done with lasers instead of live rounds, a typical tank instructor would be exposed to the firing of 120 to 240 rounds a year. Other tank soldiers estimated that in Mr. Beyer’s career, he could easily have experienced more than 3,000 blasts.

He also was hit by a roadside bomb in 2008 that left him temporarily dazed.

Research suggests that while individual blasts may cause no immediate, obvious injury, unnoticed damage can pile up over repeated exposure, causing neural connections to eventually fail.

The Army has noticed for decades that an unusual number of troops reaching middle age exhibit behavioral issues — a phenomenon it once called “old soldier syndrome.” In recent years, some researchers have suggested a connection to blast exposure.

A 40-year-old Army Reserve soldier named Robert Card, who killed 18 people in a shooting rampage in Maine in October, worked for years as a grenade instructor. His brain is being tested for an injury.

Mr. Beyer’s troubles started slowly, his wife said. His hearing grew worse each year. As a young soldier, he had loved to watch “Jeopardy!” and shout out the answers before the contestants did, but as time went on his memory became so spotty that he often had trouble finding his keys.

The decline steepened after he arrived at Fort Irwin in 2020. He complained of sharp pain in his ears, and his heart would sometimes pound as if he had sprinted a mile, even if he was sitting on a couch. His digestion was so painful that he would lay in the bathtub and cry. Brain injuries can cause dysfunction in both the heart and gut.

He started to drink heavily. At first his wife blamed alcohol for his deterioration. After he was arrested over the shoving incident in 2021, though, he stopped drinking.

“He was sober, but still not making sense,” she said. “He would talk about voices in his head. There was something clearly wrong with him.”

Dr. James P. Kelly, who led the Army’s brain injury treatment program for years and now runs a brain injury clinic at the University of Colorado, said breakdowns in soldiers with multiple combat deployments could have many causes, including post-traumatic stress disorder or mental illness unrelated to military service.

Even so, Dr. Kelly said, there are tests that can detect evidence of a brain injury, and “given this guy’s military blast exposure, that would certainly be my first thought.” He added, “It is absolutely critical to do a thorough evaluation.”

Christy Beyer said no such evaluation was ever done. After her husband was convicted at a court-martial last year, she said, he became paranoid and angry, convinced that the Army was out to get him.

Within days after becoming a civilian, he left his family without explanation, convinced that his wife was part of an imagined conspiracy against him, Ms. Beyer said.

In October, he posted rambling online rants threatening to kill specific people at his old Army post. After the incident with the knife in Northern California, the police tracked him down and arrested him at his father’s house in Petaluma, Calif., and federal prosecutors charged him in November with making interstate threats. He is also facing local charges in California.

His family hoped that he could be released from jail and put in a psychiatric hospital, but a federal judge denied him bail. Mr. Beyer is being held in a federal detention center in Los Angeles and could face years in prison.

Ms. Beyer worries that the civilian authorities, like the Army, will never take into account the damage that so many tank blasts may have done.

“In my heart and soul, I know this is an injury of war,” she said. “I just wish someone could see it.”

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