Image copyright AFP Image caption The graves of some of the villagers murdered by Bales

When soldiers heard the man who massacred Afghan villagers was Robert Bales, they weren't surprised. Newly released documents shed light on the volatile officer and the surreal environment he lived in.

US Staff Sgt Robert Bales left Camp Belambay in Kandahar Province late at night on 11 March 2012. Then he shot 16 Afghan villagers, mostly children.

The carnage has been described as "this generation's My Lai", a reference to the 1968 killing of Vietnamese civilians.

In a military court in August 2013 Bales was sentenced to life in prison with no parole.

''He liked murder,'' said a prosecutor in the courtroom in Joint Base Lewis-McChord, Washington. ''He liked the power.''

Bales had gone on four deployments to Iraq and Afghanistan, and his lawyers said he suffered from post-traumatic stress disorder. On Tuesday the US Central Command released documents about their investigation. It included 569 pages of witness testimony.

These materials were released after representatives of Tacoma News Tribune and other news organisations filed federal Freedom of Information Act requests.

Maybe the investigation wasn't complete - despite 500 pages Eugene Fidell, Yale Law School

The documents paint a portrait of Bales as an unstable individual and depict an atmosphere of drinking and drug-taking in the unit. When news of the shootings got out, according to the report, soldiers said "It must be SSG Bales."

"He's the only one crazy enough to do it," they said.

The officers who wrote the report said that discipline in the unit was sometimes lax, describing "low standards of personal conduct". The officers concluded, however, that the low standards in the unit "had no effect" on Bales' decision to carry out the crimes.

Yet legal experts say the documents provide context for the story told in the military courtroom - including a lack of leadership in the unit.

Officers who worked in the unit "took a 'hands-off' approach" on many issues, such as alcohol and drug use, according to the report.

The officers who were in charge should have been more aggressive in their efforts to guide the soldiers, say some military veterans, especially in a tough combat environment.

The report "reflects a dearth of leadership", says Gary Solis, a former military judge who now teaches law at Georgetown University. When an outfit turns an eye to smaller problems, he says, it leads to bigger problems.

Image copyright AP Image caption Soldiers described Robert Bales, shown on the left, as an "angry drunk". He has been sentenced to life in prison

Bales seemed tense and prone to violence, according to the report. He said he was unhappy in his marriage and "was behind on mortgage payments".

One person who was interviewed by the military investigators described Bales as his mentor, and said he was passionate about his work.

But he added "all it takes is the flip of a switch or a trigger for that passion to go from being used for good to being used toward something not-so-good".

One of the officers who knew Bales said he was an "angry drunk". Other soldiers said "they could smell alcohol on SSG Bales' breath from time to time when he came into their housing area" and heard him say things like: "'Hey, you're a worthless piece of white trash.'"

"He's a moody person," said another person who was interviewed by military investigators. "He would get mad if you didn't do things his way."

Image copyright AP Image caption Robert Bales, shown after his arrest, had served multiple deployments

Another soldier was more direct - "I don't think anybody really liked the guy." Yet many of the top leaders in the unit didn't seem to be aware of any problems associated with Bales.

According to the report, junior officers were reluctant to report issues because in many cases "their higher infantry leadership was committing the misconduct, and they did not feel empowered to 'jump the chain'".

The mood in the unit was tense - and surreal as a Joseph Heller novel. As the authors of the report explained, one of the commanders was known for "walking around smoking a pipe while yelling at soldiers for smoking".

Meanwhile a soldier was bit by a pet monkey at a checkpoint and had to be evacuated out of the area for medical care. The officer who wrote a memo about the incident said "it kind of paints a picture of the atmosphere in the area".

Eugene Fidell, who teaches military law at Yale Law School, says the report raises questions about the investigation that was conducted into the crimes.

"When news got out, someone said - 'It must be Bales.' Really? What was it that led somebody to say that?" says Fidell. "Maybe the investigation wasn't complete - despite 500 pages."