From the time Ned was born in northeast Nebraska, his family said, they knew he was not like other babies.
"He was more than happy to be just left lying there and didn't like to cuddle," said his grandmother, Mary Thunker, who helps to raise her daughter's five children. "He was never one to be rocked and had little interaction with other kids."
By the time Ned, not his real name, was a toddler he was having full-blown temper tantrums, kicking and screaming in public. As he got older, he attacked his mother with a baseball bat and went after his brother with a golf club.
"We always lived in fear that in the right circumstances he would get aggravated and lose control, and we never knew what he was capable of doing," said Thunker, 59. "We had a safety plan to get the other children out of the house."
After the news last week that 20-year-old Adam Lanza had killed his mother and 26 victims at Sandy Hook Elementary School in Newtown, Conn., Thunker said she thought, "Oh my god -- that could be our family."
Lanza has been described by those who knew him as socially awkward and lacking empathy -- bright, but troubled. A former high school teacher said he suffered from a condition where he felt no pain.
Mark Tambascio, a family friend, told ABC he believed Lanza's mother, Nancy, had become increasingly concerned in the last few months about her son's emotional and behavioral issues.
Thunker said she understands how difficult it is to deal with a troubled child and to get proper treatment in a mental health care system that is fragmented and not well funded.
Today, Ned is 15 and is stable and living at home after two hospitalizations -- one at only age 7 -- and more than a year of residential treatment at Boys Town.
He was eventually diagnosed with anxiety, attention deficit hyperactivity disorder (ADHD), opposition defiance disorder and some bipolar tendencies, which "opened the door for services," Thunker said.
But she said the family nearly had to give up custody of Ned to the state to get the care he needed.
The National Institute of Mental Health reports that about 26 percent of the general population has sufficient symptoms to be diagnosed with a mental health disorder.
About half of all lifetime cases of mental illness begin at by the age of 14, and decades often lapse before these people get treatment, according to research supported by the NIMH. By the time they reach adulthood, their disorders are more severe and not as easy to treat, leading to the development of co-occurring conditions, research finds.
But knowing when social awkwardness is a personality disorder that might lead to violence is hard to predict, say psychologists. And often the signs aren't as pronounced as they were in the Thunker family.
"We have learned whether or not they have a diagnosable disorder, there are precursors or signs going on," said Kenneth Dodge, director of public policy at the Center for Child and Family Policy at Duke University.
"But these single events [like the Sandy Hook massacre] are very difficult to predict and we don't want to beat ourselves and our neighbors up that we didn't discover it or know it ahead of time," he said
Susan Klebold, whose son Dylan, with Eric Harris, slaughtered 13 students and teachers at Columbine High School in 1999, said she missed many signs of his depression and suicidal thoughts.