Criminal allegations aside, the return of the Fifth Stryker Brigade to Joint Base Lewis-McChord last summer would still have been a somber one. During its year in Afghanistan, 35 soldiers were killed and around 230 wounded. The toll represents one of the highest casualty rates of any brigade deployment to the war. The Fifth Brigade, with roughly 4,000 soldiers, comprised seven battalions; each battalion had about four companies, each company about four platoons. While most of these units shifted continually among the southern provinces — from Kandahar to Helmand to Zabul — Third Platoon, Bravo Company, was from the outset stationed west of Kandahar, at a forward operating base called Ramrod.

The war was not what they expected. By September 2009, two months after their arrival, the soldiers of the Third Platoon were conducting two missions a day to villages and compounds scattered throughout an area of operations many felt was too vast for them to influence. “We went to the same town every few weeks,” one of the defendants, Pfc. Andrew Holmes, said through his lawyer in response to my written questions. “We did not seem to have any familiarity with the locals. The unit was always making empty promises on how often we would return. On top of that, there were I.E.D.’s everywhere.” Several members of the platoon said that the prevalence of improvised explosive devices and near absence of actual combatants engendered frustration. “A lot of guys felt gypped,” another soldier told me. “All the other units have these great stories about firefights, then here we are, we’re not getting anything. We had to sit there and just wait to get blown up.” (Three soldiers from the Third Platoon, on condition of anonymity, were willing to discuss their deployment and the crimes with me. It should be said that the allegiances and grievances that no doubt arose during their year in Afghanistan, and that can have only worsened during subsequent investigations and legal proceedings, might very well have created biases.)

In November, one of the vehicles in a Third Platoon convoy passed over a pressure-plate bomb that exploded directly underneath its driver, Sgt. Robert Samuel, breaking several of his bones and completely severing his left leg. Within the platoon, Samuel had been an extremely popular noncommissioned officer and squad leader, particularly among the younger soldiers. “Once that happened, it was kind of a big reality check,” the soldier quoted above told me. By this time, low morale had already led to widespread misconduct, and the majority of the Third Platoon (at least 15 soldiers, according to Army investigations) was getting stoned several times a week. “It’s commonplace,” the same soldier said; he added that no one was forced to submit to urine tests. “All the local nationals that work on the F.O.B., they all get high. All it takes is you trade them a porno mag, and they give you an ounce of hash.”

Brig. Gen. Stephen Twitty, the investigating officer who, in the wake of the murders, was assigned to conduct a command-climate inquiry into the Fifth Brigade, would later find that the Third Platoon “had significantly lower standards and discipline than other units.” In his report — the same one Mestrovic referred to, and which The New York Times has reviewed — Twitty writes that both the platoon leader, Lt. Roman Ligsay, and his noncommissioned counterpart, Sgt. First Class Julio Bruno, were regarded as “weak leaders, lacking confidence, self-serving, focusing on wanting to be liked by the soldiers so failing to enforce standards, and not engaged in the platoon’s daily activities.” The soldiers I spoke with agreed with this assessment.

Sgt. Samuel’s replacement was Calvin Gibbs, a physically imposing staff sergeant from Billings, Mont. At 25, Gibbs had already served two tours — one in Iraq, one in Afghanistan — and he quickly demonstrated tactical instincts that earned him the esteem of his seniors, who often placed him in the lead. His confidence and aggression made a strong impression on the other soldiers, especially those with little combat experience. “He’s the kind of guy, he would’ve been on the poster for GoArmy.com,” one soldier told me. “He’s what you want a soldier to look like, act like, speak like. He’s like the epitome of soldier.” Another said: “Gibbs was just a really personable guy. Easy to like, funny, in shape. He just had some sinister hobbies.”

Jeremy Morlock, a corporal at the time, would later tell investigators that a week or two after joining them at Ramrod, Gibbs began talking about “some of the stuff that he had gotten away with in Iraq.” As an example, Morlock said, Gibbs related having killed an Iraqi family driving in a car. (Gibbs has not been charged with any crime related to his service in Iraq. Army investigators failed to find any witnesses to this event; the medic attached to Gibbs’s old platoon said in a sworn statement that he had heard that Gibbs and others had fired on a swerving vehicle one night, killing a man, woman and child.) Not long after, Morlock further claims, Gibbs “tossed out this scenario” in which it would be possible to kill an unarmed Afghan in such a way that would make it appear to have been a legitimate engagement. During the deployment, according to Morlock, Gibbs had acquired several fragmentation grenades off the books. The “scenario” would involve shooting an Afghan, detonating one of the grenades and later explaining to superiors that the explosive had been thrown by the victim.

Morlock and another soldier accused of murder, Adam Winfield, have characterized Gibbs as a sociopath who orchestrated the killings, and Winfield further claims Gibbs used his rank as a noncommissioned officer to coerce him into participating. The “kill team” moniker, instantly and unanimously adopted by the news media, comes from a leaked video interview in which Winfield tells an Army special agent that Gibbs “thought I was weak and I’m not good enough to be on his quote-unquote ‘kill team.’ Then he asked me if I would be in.” Gibbs, from the beginning, has denied any wrongdoing.