Loughner -- the 22-year-old charged with the attempted assassination of Rep. Gabrielle Giffords -- has refused to speak with investigators since allegedly opening fire at a political event Saturday.
Online videos with puzzling prose about government, terrorism and sleep, along with descriptions from people who knew Loughner, paint the picture of a once-friendly student who, over time, lost touch with reality. But Loughner's reported "obsession" with Congresswoman Giffords and frustration over political issues add complexity to an action most would consider insane.
"You don't have to be nuts to do this," said Dave Cullen, author of "Columbine" -- a book documenting the 1999 shooting at Columbine High School in Colorado. "You have to be troubled in some way."
According to Cullen, few killers are actually insane -- meaning they can't distinguish between what's real and what's imaginary. Rather, killers with political motivation, such as Timothy McVeigh, or a personal agenda, such as Eric Harris at Columbine, understand what they're doing even though others may not.
"[Loughner] could have had some personal animosity with Congresswoman Giffords, but more likely he had a problem with government and she, as his Congressperson, is a symbol of that," said Cullen.
A signed note found in a safe at the Tucson home he shared with his parents suggests Loughner planned his attack. And a form letter from Giffords' office thanking him for attending an event similar to Saturday's "Congress on your Corner" implies the two had met before.
But based on Loughner's past -- including several run-ins with campus police and a request for clearance by a mental health professional from Pima Country Community College -- mental illness seems likely, Cullen said.
Theodore Kaczynski, the so-called Unabomber, was ultimately diagnosed with paranoid schizophrenia -- a condition marked by delusional, mistrustful thoughts, according to the National Institutes of Health.
Tragedy in Tucson: Could Loughner Suffer from Paranoid Schizophrenia?
Schizophrenia strikes about one percent of the adult population, usually between ages 18-22, according to Dr. Paul Ragan, senior consulting psychiatrist at Vanderbilt University in Nashville, Tenn. And the paranoid type is the most common.
Loughner's former classmate Tong Shan told ABC News, "He was a good person that just somehow changed so much … I don't know what the hell happened to him."
Based on Shan's observation and given Loughner's age, Ragan says Loughner, like Kaczynski, may have suffered from paranoid schizophrenia.
In an interview with Mother Jones, Loughner's friend Bryce Tierny said Loughner was obsessed with "lucid dreaming" -- dreaming that can be controlled like an alternate reality. Loughner became "more interested in this world than our reality," Tierny told Mother Jones.
Tierny also said Loughner kept a dream journal. "You want to know what goes on in Jared Loughner's mind, there's a dream journal that will tell you everything," he said.