KABUL, Afghanistan — The U.S. military bans alcohol for its troops in Afghanistan, but that doesn’t stop some soldiers from having a bottle or two stowed away in their gear — a fact highlighted by investigators’ probe into whether alcohol played a role when a U.S. sergeant allegedly carried out a killing spree that left 16 Afghans dead.

U.S. investigators have determined that the suspect had been drinking alcohol prior to leaving the base the night of the attack, a senior U.S. defense official said Friday. How much of a role alcohol played in the attack is still under investigation, the official said.

Like many rules in a war zone, the U.S. military’s General Order No. 1 forbidding alcohol in both Afghanistan and Iraq is not always followed to the letter. Even in these strictly Muslim countries, there are ways to access liquor. Amid the tight-knit camaraderie of a stressful battlefield, officers sometimes turn a blind eye — or even partake themselves.

In Iraq, booze was easy to come by in Baghdad’s Green Zone and on some bases. In Afghanistan, soldiers from many other NATO countries are allowed to imbibe. That means there’s some “alcohol spillover” to American troops on large multinational bases. In both countries, foreign contractors dealing with the U.S. military — most of whom were not covered by the order — bring in their own supplies and are a source that soldiers can turn to.

German troops stationed in Afghanistan are allowed two beers a day at their main base in northern Afghanistan, but not at smaller camps. In Kabul, one military base that mainly houses European troops boasts two liquor stores.

On Kandahar Air Field, the main international base in southern Afghanistan, Canadian forces used to have regular beer nights before they pulled their forces out this past summer. Each person was limited to two beers and half a bottle of wine.

At these large installations, U.S. soldiers also sometimes manage to get alcohol in packages sent by family and friends, often hidden in other types of bottles.

Finding alcohol is more difficult in more remote areas of the country or on smaller bases, like the one from which the soldier allegedly slipped out to start his shooting spree.

Some rural Afghans make homemade wine out of raisins but in general few Afghans drink — so alcohol would have to be brought in by soldiers or brewed using local ingredients.

A senior U.S. defense official said earlier this week that investigators had found alcohol at the soldier’s base, Camp Belambai in Panjwai district.

The suspect — identified by his attorney as 38-year-old Army Staff Sgt. Robert Bales — is said by military officials to have left the base at 3 a.m. Sunday, walking to two nearby villages where they say he barged into homes and opened fire, killing 16 people, including nine children.

The U.S. military’s General Order No. 1 forbids “possessing, consuming, introducing, purchasing, selling, transferring, or manufacturing any alcoholic beverage” in both Iraq and Afghanistan. Soldiers found violating the order can face discharge or criminal charges.

War zone deployments have not always been so ascetic, of course. During the Vietnam era, drinking was allowed and both drinking and drug use were common among soldiers. At that time, raucous, alcohol-fueled nights out on the town in Saigon were routine.

But as men returned from Vietnam as alcoholics and drug addicts, the military started to revisit its substance abuse policy. In 1971, the government required all military branches to identify substance abusers and provide treatment and rehabilitation. More than 20 percent of soldiers tested positive for drugs when the Army started screenings, according to U.S. military figures.

Then, when the U.S. sent peacekeepers to the Balkans in the 1990s, commanders went further and banned all alcohol for the deployed troops.

Because of the risks of sneaking in alcohol, most U.S. soldiers simply get by without it during their tours in Afghanistan. They buy nonalcoholic beer in stores on base for the familiar taste. Those looking for a buzz take up smoking or chewing tobacco.

There are many more stories of U.S. soldiers on small outposts abusing prescription drugs or smoking easily available hashish. Drug abuse is a rampant problem in the Afghan army and U.S. soldiers have been known to start smoking up alongside their Afghan counterparts.

On remote, hilly outposts, soldiers often make it through the night with sleeping pills and joints get passed around. Some become addicts. Most say they’re doing what they have to, facing a morning that could bring a new firefight or a roadside bombing on their patrol.

In Iraq in 2006, a U.S. soldier stationed near Mahmoudiya raped and killed a 14-year-old Iraqi girl and killed her parents and sister. Steven Dale Green, a former 101st Airborne soldier, was sentenced to five life terms for the crime. He said later that drugs and alcohol were prevalent at his checkpoint south of Baghdad. He also said that he had been taking a mood-regulating drug to help him deal with the traumatic events he’d seen.

Green said by the time he committed the murders he had seen so much violence and so many people killed that he had stopped thinking of Iraqi civilians as humans.

“I was crazy,” Green said in 2010 interview from federal prison in Tucson, Arizona. “I was just all the way out there. I didn’t think I was going to live.”