Colorado massacre suspect James Holmes gave no outward signs of mental illness or violent delusions, and mental experts said that is common among mass murderers.

Before Friday's massacre, Holmes had no previous brushes with the law beyond a single traffic violation. Dr. Marisa Randazzo, a psychologist who studies targeted violence, told ABC's "Good Morning America" that, surprisingly, a clean criminal record is not uncommon for people who commit acts of mass violence.

"In most of these cases, these are not what you would call a psychopath or a sociopath, as hard as it may be to believe," Randazzo said. "These are often folks who often up onto this point have been functioning fairly normally but went through a series of events, a series of losses, ended up in absolute despair or desperation."

Other psychologists told ABC News it's likely that Holmes was living in an alternate reality driven by delusions, which may have fueled him as he bought weapons, 6,000 rounds of ammunition and riot gear in the months before Friday's attack.

More details will likely come as investigators delve into Holmes' recent past. But by most estimations so far, nothing about his early life was out of the ordinary. He grew up in San Diego, was a bright student interested in science and enrolled in a neurosciences doctorate program at the University of Colorado at Denver in 2011 before withdrawing in June.

No one who knew him has said he displayed any signs of abnormality. Randazzo told GMA that doesn't mean he isn't suffering from mental illness.

"One thing we do know about this age group, he's 24, is that sometimes major mental illnesses, sometimes involving delusions, will develop in this age group," she said.

Upon his arrest shortly after the shootings on Friday morning, Holmes allegedly told police that "he was the Joker," a law enforcement official told ABC News, and he had dyed his hair red.

ABC News reported Sunday on "This Week" that police also found a Batman poster and Batman mask in Holmes' apartment.