LAKELAND, Fla. (Tribune News Service) — What explanation can there be for an act of unfathomable cruelty?

As news spread Sunday about a pair of shootings in North Lakeland that left four dead, attention turned to the suspect, Bryan James Riley of Brandon. Polk County Sheriff Grady Judd said Riley surrendered to deputies Sunday morning after being wounded in a shootout with officers.

After Riley emerged from the house where he had taken cover, PCSO deputies found four bodies inside, including those of a 33-year-old woman and her infant son. In describing the slaughter and the shooting of an 11-year-old girl who survived, Judd referred to Riley as "evil."

Riley confessed to the killings but suggested he was mentally ill, Judd said.

The sheriff revealed that Riley, 33, is a U.S. Marine Corps veteran and that his girlfriend said he suffers from post-traumatic stress disorder, or PTSD. Judd said that Riley told investigators he was high on methamphetamines at the time of the rampage.

Riley began active duty with the Marine Corps in 2007, when he was 18, according to military records provided by the Polk County Sheriff's Office. Rated as a sharpshooter, he was deployed to Iraq for seven months and later served a tour in Afghanistan from December 2009 to July 2010.

Riley received an honorable discharge in March 2011 with the rank of corporal and served in the USMC Reserves until June 2014, finishing as a Motor Vehicle Operator at Camp Lejeune in North Carolina. Authorities have not yet said whether Riley sustained physical injuries during his deployments.

While it's natural to seek an obvious explanation for Riley's alleged acts, focusing on his diagnosis of PTSD would be a mistake, said another veteran, Dan Jarvis.

Jarvis is founder and president of 22Zero, a Lakeland-based nonprofit that provides therapies for veterans enduring symptoms of PTSD. The name reflects the reality that 22 veterans a day kill themselves and the goal of reducing that number to zero.

Jarvis, a retired Army sergeant first class, said veterans with PTSD are not typically violent.

"Most of the veterans that have PTSD are more info self-harm," he said.

Jarvis said Riley's apparent use of methamphetamines seem a more likely cause of Riley's alleged actions. Riley allegedly drove into North Lakeland and approached strangers at their properties, saying he had been sent by God. Riley warned a man that his daughter, Amber, planned to kill herself, Judd said, though the man had no daughter by that name.

After his surrender, Riley told law-enforcement officers that he was taking methamphetamines, Judd said.

"I would say that the guy's methamphetamine use was probably a little bit more of a reason why he acted in psychotic manner because that's exactly what methamphetamines do to people when they go on benders, and the sheriff knows that," Jarvis said. "I would not say that the PTSD is the reason for it. I would say his drug use."

Extended use of meth induces an altered sense of reality, Jarvis said.

"If you're on a four- or five-day bender, you go into a psychotic state," he said. "You'll hallucinate. You'll see things, hear things, that aren't there, almost like what a schizophrenic might see or hear."

Jarvis is not a trained mental-health counselor. His organization uses a peer-support model, and he said most of the 130 members of 22Zero's support staff are veterans and first responders who have received training in the program's methods.

The National Institute on Drug Abuse, part of the federal National Institutes of Health, lists visual and auditory hallucinations as potential effects of meth use, along with delusions and paranoia.

PTSD is a disorder that can develop after a person experiences trauma. Some 11 million veterans endure the effects of PTSD, according to the Veterans Administration, and the condition plagues as many as 20% of those who served in Iraq and Afghanistan.

In PTSD, intense or disturbing thoughts and feelings associated with the trauma persist after the event is over. People with PTSD can experience nightmares or sleep disturbance, and certain situations can prompt memories of the trauma that trigger panic or other negative reactions.

The American Psychiatric Association calls PTSD a psychological disorder, though Jarvis said he considers it a neurological disorder. He said the condition causes the limbic system, which regulates the fight-or-flight response, to "hijack" the brain.

"The brain is neurologically predisposed to the fight-or-flight part of PTSD because what happens is when a person is exposed to a traumatic event, the brain neurologically connects an emotion to the event," Jarvis said. "And when they don't process it appropriately, either through sleep deprivation or self-medication, the brain will never process out those emotions."

Jarvis' organization uses two main methods to treat PTSD by addressing its underlying sources. One seeks to disconnect the emotions from the traumatic event, and the other explores subconscious sources of anger or other negative emotions.

Jarvis said he hopes the details of the North Lakeland massacre don't lead others to assume that anyone with PTSD is likely to commit violence.

"You can't blanket an entire group of people on the actions of one individual," he said. "Will people have that fear? I don't know. I hope not."

Afghanistan has been in the news recently, as the withdrawal of Americans troops after a 20-year presence has led to scenes of chaos. Jarvis wouldn't say if that could have influenced Riley, but he said the news has affected other veterans who served in Afghanistan.

"It's triggering a lot of veterans," he said. "A lot of veterans are asking the questions, 'Why did I serve? Why did I risk everything? Why did I lose my buddies to have the administration just kind of walk away the way they did?'"

He added: "But nothing like that would ever excuse what that guy did. He needs to be held accountable for his actions."

More information about local treatment for veterans and first responders with PTSD can be found at https://22zero.org.

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