The aftermath of last week's killing of 16 Afghans has prompted a flurry of speculation into the mind of 38-year old U.S. combat staff sergeant Robert Bales. In particular, the injuries to it.

Traumatic brain injuries are so common among today's troops that the military has spent over $42 million for a test to detect them, a test that Bales most likely took before his final deployment to Afghanistan. The problem is, that test has failed miserably.

More than a million soldiers have taken the 20 minute computerized test, known as the Automated Neuropsychological Assessment Metrics, or ANAM test. But as we reported last year in a ProPublica investigation, the test has been heavily criticized as an ineffective tool to detect brain injuries.

Many news outlets, including the New York Times, have cited military officials saying Bales was treated for a traumatic brain injury during his past deployments in Iraq. Bales was reportedly injured in Iraq when his vehicle rolled over. The Army Medical Command would not comment on any specifics of Bales' medical history or testing, but spokeswoman Maria Tolleson said that Joint Base Lewis McChord, where Bales was stationed, was fully operational with the ANAM testing program.

"It would be expected that a deploying Army service member from that base would have a pre-deployment cognitive baseline completed," said Tolleson.

Problems have plagued the test since its introduction. Critics charge the military chose the test through a biased selection process and then ignored years of warnings that the test was fraught with problems. They also say the military has not administered the test properly.

Soldiers are meant to take the test twice 2013 once before deployment and then again after a suspected head injury. Soldiers must answer a series of questions that score basic thinking abilities such as reaction time, short-term memory and learning speed. In theory, the initial test serves as a baseline to compare the results of the second test; a discrepancy signals a possible injury and the need for more evaluation.

But the test 2013 which a former Army surgeon general has called no better than a "coin flip" 2013 is rarely implemented that way. The Army was so unconvinced of the test's accuracy that it issued an order not to send soldiers with a troublesome score for further medical evaluation.

While there is no scientific consensus on the best test for traumatic brain injuries, alternatives do exist. Both the Army Special Forces and the National Hockey League chose a different test, called ImPact. Researchers are also developing new technologies to detect brain injuries, but right now the ANAM test remains the prime military TBI test.

The precise nature of Robert Bales' brain injuries and subsequent testing is one issue, but linking his injuries to his outburst of violence is another matter altogether.

Scientists have not established any clear-cut connection between traumatic brain injures and later violence. This article from Wired explores some of the most recent studies on the topic, while the Los Angeles Times breaks down the many interacting factors that make drawing a clear line from injury to violence nearly impossible.

This story was originally published by the independent, non-profit newsroom ProPublica.