U.S. Army Staff Sgt. Robert Bales, who went on a killing spree in Kandahar, Afghanistan, last year, may have been under the influence of a number of drugs, including steroids and the anti-malarial mefloquine hydrochloride, which combined with existing psychological trauma, leading to the killing of 16 civilians and others who were injured.

Mefloquine was developed by the U.S. military as preventative treatment for malaria. It's been shown to cause varying neurological side effects five to 10 percent of the time, and its labels warn against anyone who has suffered a traumatic brain injury (TBI) or seizure from taking it, ABC News reported.

Was Mefloquine Responsible?

But Bales, who suffered head and foot injuries while deployed in Iraq, and who could have been traumatized from a roadside bomb attack that claimed a fellow soldier's leg, was still given the drug. Bales' lawyer, John Henry Browne, said that he took mefloquine while in Iraq but that his records in Afghanistan were incomplete.

"We know that he was given Lariam (brand-name mefloquine) while in Iraq," Browne told The Seattle Times. "We just don't have a complete set of medical records for that period [in Afghanistan]. He can't help us. He just says he took 'whatever they gave me.'"

The subject of mefloquine emerged when a Food and Drug Administration (FDA) adverse-event report from March 2012 spoke of a soldier-patient in the U.S. Army who "developed homicidal behavior and led to homicide killing 17 Afghanis."

According to the report, which didn't say Bales' name, and was written by an anonymous pharmacist, "a patient of unknown demographics was treated with mefloquine hydrochloride for an unknown indication," and his medical history included a TBI.

"It was reported that this patient was administered mefloquine in direct contradiction to U.S. military rules that mefloquine should not be given to soldiers who had suffered TBI due to its propensity to cross blood brain barriers inciting psychotic, homicidal, or suicidal behavior."

In 2009, the Defense Department made mefloquine a third choice for malaria prevention, behind two other medications, after concerns arose.

The report put an unexpected twist on the Bales case, since it sounds like it could be him, however, it comes from an anonymous source — even though the document is real — and instead of listing 16 as dead, it lists 17. But according to Time, the original number was 17, and it was changed after an investigation found it was actually 16. Still, at least one expert remains skeptical.

"Tens of millions of people take it," Dr. David Sullivan, an infectious disease specialist at Johns Hopkins Malaria Research Institute in Baltimore, told ABC. "Honestly, you cannot implicate any one thing. To put it all on mefloquine is not fair. [Bales] already has a predisposition because of traumatic brain injury and he has taken this drug in a stressful situation. You have to put it in context here. ... But you can't exclude it."

Bales, 39, who is a father of two from Washington State, also admitted during his trial that he had used illegal steroids to improve muscle tone and recovery time from missions. He said that it also increased his irritability and anger, according to the Daily Mail.

Sgt. Bales Killing Spree

The stress, mefloquine, steroids, and even alcohol might have culminated into a dangerous mixture that led Bales to murder the civilians. According to an Army Corporal who testified at a pre-trial hearing last fall. He said that the two of them, along with a third soldier, were watching the Denzel Washington film "Man on Fire" while drinking whiskey.

Bales admitted to the killings that left 16 civilians dead and six injured. Nine of those dead were children and many of the bodies were piled-up and burned.

Video footage showed Bales returning to base with a bed sheet — or throw rug — tied around his neck like a cape when he was told to drop his weapons by other soldiers, and arrested. Corporal David Godwin, reported Bales repeating the words, "I thought I was doing the right thing," and "It's bad. It's bad. It's really bad."

Bales plead guilty to 16 counts of murder, six counts of attempted murder, and seven counts of assault. He did not plead guilty, however, to a charge of disrupting the investigation by destroying a laptop computer. Because of his guilty plea, he won't face the death penalty, however, he still faces life with or without the possibility of parole. Sentencing is scheduled for August.