According to a military statement, Leah King died Feb. 15 in Bennitt's room from a lethal combination of Xanax and the pain-killer oxymorphone, also known as Opana -- all allegedly supplied by Bennitt.
According to an accompanying military charge sheet, released by Ft. Lewis, King "had a propensity to abuse controlled substances."
King was dead when officials responded to Bennitt's room and another unidentified teenager, also 16, was found unconscious but survived, after being transported to nearby Madigan Army Medical Center.
In addition to the manslaughter charge, Bennitt was also charged with wrongful use and distribution of controlled substances and conspiracy to use them.
He's accused of distributing Xanax and Opana as well as Percocet and marijuana. Bennitt faces up to 82 years in prison, a demotion in rank, forfeiture of pay and a dishonorable discharge if convicted. .
The families of Bennitt, who is from Rolling Prairie, Ind., and King could not be reached for comment.
Investigators found that some of the drugs appeared to have been crushed and "inhaled in powder form."
In accordance with military police, an investigating officer will be assigned to conduct a hearing equivalent to a civilian grand jury.
Asked if King was known to have visited the base before, Ft. Lewis spokesman Joe Kubistek said he couldn't release more information beyond what was in the Army's statement and the charge sheet for Bennitt, or say if other soldiers could face related charges. "The investigation is still open and ongoing," he said.
But Fort Lewis has amended its visitor policies since King's death. Soldiers and Department of Defense employees are still able to sponsor visitors on base, including minors, but those minors must now be signed in at the visitor's center.
"The staff at the visitor's center will screen minors and sponsors -- denying access to those who do not appear to have a legitimate reason to visit Fort Lewis," the Army's statement said.
Random checks of vehicles and barracks have also been stepped up to prevent unauthorized minors from sneaking onto the base.
Amid numerous posts offering condolences on King's MySpace page, a poster using the name Heather identified herself as Leah's sister and wrote shortly after her death, "just so everyone knows, we have very few details about what happened to Leah. But what we know, we are not supposed to share just yet."
The post, attributed to "Leah's Family," also requests that other family members and friends not make comments to the media.
ABC News cannot independently confirm the claims made on the MySpace page.
In the "About me" portion of her MySpace page, King had posted a personal statement that read: "I'm Leah. I'm annoying. rude. A bitch. stupid. A slut. A nobody. and any other label there is."
Her relationship status says "single," but local media reports have said she was dating an unidentifed soldier based at Ft. Lewis.
Two former soldiers told ABC News Seattle affiliate KOMO-TV it isn't difficult to sneak visitors onto the base and said the barracks have been known to draw curious young girls.
"It can certainly be attractive, an attractive environment with a bunch of soldiers who are older and have access to alcohol and drugs oftentimes," said Seth Manzel, who spent four years as a soldier, often working guard duty on the base.
Manzel told KOMO that the effects of drugs and alcohol, along with the overwhelming stress of having been deployed to war zones and the mental toll that can take, can make men do things they shouldn't.
"There were times when I walked in on things that were pretty atrocious. At one point a girl got branded. She was a willing participant, but she was very drunk and not in a position to make that decision," he said.
The teenager's death was just the latest unsavory headline connected with Fort Lewis and its soldiers.
The last several months have seen a number military personnel charged with serious, violent crimes, including several incidents of alleged assault, a homicide investigation involving a sergeant at the base and another awaiting trial on charges of kidnapping, torture and rape. Fort Lewis spokesman Joe Piek told ABCNews.com last month that military officials were well aware of the legal troubles involving soldiers at Fort Lewis.
"That's actually something the command is taking a look at right now," he said. The Vancouver Columbian newspaper reported last month that a 23-year-old Fort Lewis soldier was arrested on driving under the influence and assault charges after a road rage incident over the weekend.
And according to various Seattle-area media reports, three Fort Lewis-based soldiers were arrested on assault and robbery charges in recent weeks in connection with two incidents last month when University of Washington students were robbed at gunpoint and assaulted. In January, Fort Lewis Sgt. Nathan Ryan Smith, 29, was charged with kidnapping, raping and torturing two women after he was reported AWOL.
According to a probable cause statement from Pierce County prosecutors, after Smith allegedly raped one woman he told her, "I have killed several people in Iraq. I'm crazy in the head, and if I get caught by police I will come looking for you and kill you." The News Tribune in Tacoma reported that he pleaded not guilty.
And in November, a sergeant was arrested in connection with the fatal shooting of his friend after a day of drinking. The medical examiner's office ruled the death a homicide, according to the News Tribune.
Dr. Fred Sautter, director of the family mental health program at the Southeast Louisiana Veterans Health Care System in New Orleans, said soldiers sometimes face a bevy of trickledown problems stemming from their military service.
Post-traumatic stress disorder -- long flagged as one of the most pressing problems for soldiers deployed as part of the war on terror -- can cause a state of "hyper arousal" that makes solders irritable, startle easily and get "really revved up," according to Sautter.
But while PTSD may explain certain emotional and even violent outbursts, it may not fit in the most recent incident.
"Having a 16-year-old girl in the barracks doesn't really fit," Sautter said. "PTSD doesn't make you have young girls in the barracks."
Substance abuse resulting from PTSD can be an issue for military members returning from combat.
Sautter said that about 17 percent of soldiers coming back from Iraq and Afghanistan show symptoms of PTSD. And data shows that, of those people, 65 to 70 percent may develop substance abuse problems.
"You have people who are really agitated and hyper vigilant and really on an adrenaline high," Sautter said. Drugs and alcohol are often ways for soldiers to calm down or control that high.
But the drugs then cause a whole new set of problems, including poor judgment and impulsivity.
Pentagon-based Army spokesman Lt. Colonel Nathan Banks told ABCNews.com that arrests and convictions of Army soldiers aren't broken down by base, and no one place is dubbed a "problem child."
"We could possibly have some issues at other installations. Some just stand out more than others," he said.
Banks said the Army's thoughts and prayers are with the family of the teenage girls.
"Any incident that is serious, we are aware of it and we are very concerned," he said.
Fort Lewis is currently home to about 30,000 soldiers with another 4,000 deployed, Piek said, mostly to Iraq and Afghanistan.
"It's very unfortunate that we have some soldiers who are doing things like that out in the community because it sheds a bad light on the much larger community," Piek said of the many more men and women who take the Army's moral code very seriously. "It's still not indicative of the majority that are serving."
And the communities that surround the base, he said, expect better from America's soldiers.
The Associated Press and ABC News Research's Suzanne Bernard contributed to this story.
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