What spurred one American soldier to allegedly massacre 16 Afghan civilians earlier this week? That's a complicated question, and one that could take military investigators months or years to figure out.

What is known, among sparse details, is that this soldier suffered a traumatic brain injury (TBI). That factor will no doubt play a role in the military's investigation, and it offers yet another reminder of the military's awful track record in diagnosing and treating that ailment, widely known as one of the signature wounds of the wars in Iraq and Afghanistan.

In my years reporting on TBIs among soldiers and vets, it's become increasingly apparent that problems in TBI management start even before a soldier deploys, and persist – often with devastating results – long after he or she comes home. "We got hit a lot of times in Iraq, [so] I definitely got rattled around," Staff Sgt. Victor Medina, a soldier afflicted with TBI, told me in 2010. "It wasn't until the fourth time we got hit, and I blacked out, that anyone took me to get looked at."

Medina later relied on alternative therapies like massage and acupuncture, in part because it was so tough for him to get adequate care at Fort Bliss, where he was stationed. "One doctor told me I was making it all up," he said, of symptoms like stuttering and blinding headaches that followed the TBI he suffered during a massive IED explosion in 2009.

He's hardly the only soldier to feel inadequately cared for by military docs.

Indeed, the military's mismanagement of TBIs over the past decade is nothing short of astonishing. Military docs have failed to diagnose soldiers who showed clear symptoms of injury. Potentially thousands of medical diagnoses for TBI have been all-out lost. Soldiers sometimes wait months to start TBI treatment. Most importantly, scientists still don't even know what TBIs do to a person's body, brain or long-term mental health.

An estimated 200,000 soldiers have suffered from a TBI over the past decade. At least, as far as the military can tell. Even after the Pentagon in 2007 injected $1.7 billion into better management of TBIs, military docs still can't accurately diagnose the injuries during a soldier's deployment or upon his or her return to base.

Right now, the military typically uses a three-phase screening test to spot TBIs in personnel. One baseline test is taken before deployment, another after a possible TBI has been sustained during deployment, and a third when a soldier returns home.

But, as highlighted in a 2010 ProPublica/NPR investigation, those tests are hopelessly flawed: One screening test failed to spot 40 percent of TBI cases among military patients. Another was described as "basically a coin flip," by Lt. Gen. Eric Schoomaker, the former Army Surgeon General.

And a better replacement has yet to be found. Both the Army and the Navy have touted breakthrough TBI diagnostic measures – the Army's is a blood test, the Navy's is an online exam – that later proved to be wildly overhyped. The Navy's test actually doled out more false positives than the military's current screening program, while the Army's blood test hasn't even undergone clinical trial.

Even if a soldier is diagnosed with a TBI, there's no guarantee that the diagnosis will make it into his permanent medical records. That's because the handheld systems used overseas to track injuries often broke. In other instances, they couldn't connect to adequate bandwidth to transmit a given diagnosis. As a result, plenty of TBI diagnoses have been lost: One unpublished Army survey, reviewed by ProPublica, noted that 75 percent of soldiers suffering from a brain injury had no record of ever sustaining one.

Treatment for TBI among soldiers is yet another major problem. Over the past decade, media reports that soldiers with TBIs were forced to fight for care have trickled out with frightening consistency. The Washington Post's 2007 investigative series on Walter Reed, for example, dug up myriad instances of neglect and inadequate treatment of ailing soldiers, including those with brain injuries.

In the years to follow, soldiers with TBI have continued to express frustrations about many of the same problems. Indeed, military medical records show that soldiers still often wait weeks or months to undergo therapy. This, even though a panel of experts convened by the Pentagon recommended that a patient's TBI treatment be initiated as soon as possible, so as to minimize long-term damage.

Madigan Medical Center, where the solder alleged to have opened fire on civilians this week was treated, is no exception to such lapses. A recent Army investigation, for example, concluded that doctors had inappropriately downgraded the PTSD diagnoses of 280 soldiers, rendering them eligible for fewer benefits and less treatment. And of course, doctors at Madigan had the same problems with diagnosis and treatment as colleagues elsewhere – a lack of effective tools and concrete knowledge.

These gaps in knowledge even extend beyond diagnosis and treatment, right down to the physiology of TBI itself. Even the symptoms of TBI can be incredibly diverse, tough to spot, and sometimes emerge months after an injury is sustained. The most well-known symptoms include confusion, headaches and difficulty reading or speaking. Other soldiers, however, suffer nausea and fatigue. And there's also a clear link between TBIs and violent, aggressive behavior – which is frequently noted in the military's own TBI treatment guidelines.

In addition to a puzzling, scattershot array of symptoms, Pentagon-funded scientists still aren't sure how brain injuries affect the brain, both in the short and long term. Researchers even suspect – though they aren't sure – that TBIs "prime" the brain for post-traumatic stress disorder, the other signature wound of this decade's wars. PTSD symptoms, of course, often include problems like hyper-arousal, irritability or outbursts of rage.

With so many questions that persist about how to treat a TBI, it's hard to know when a soldier suffering from one can be safely redeployed – like the Staff Sergeant who allegedly went on this week's rampage – or if they even can at all.

"I am trying to find out basically whether there was a premature 'OK' on this guy," Rep. Bill Pascrell, founder of a congressional task force on brain injuries, told Reuters today. "If this soldier fell through the cracks, does that mean others have?"

It's not known yet how big a role this soldier's brain injury played in this week's tragedy. But there's no question that for 10 years, the military has failed to adequate diagnose and treat these ailments. And that other soldiers with TBIs have, indeed, fallen through the cracks.

UPDATE: Over at Time's Battleland, Mark Thompson has unearthed some interesting details about the military's policy of "deferring" mental health diagnoses until a soldier returns from deployment. "'The commander, the patient, the doctor – all search for a fix that will keep the soldier in the fight...As a result, doctors often compromise — they avoid making a diagnosis that will destroy a soldier’s career.'" Obviously, turning a blind eye to certain symptoms (delusions, for example) could have very dangerous implications.