(CBS) -- The Newtown school shooting is "fundamentally different" from most mass shootings in this country because it "started with a matricide, of a boy killing his mother," according to attorney Paul Mones, who specializes in defending teens and young adults who have killed their parents.
Newtown shooter Adam Lanza reportedly shot his mother four times in the face while she was in bed before driving to Sandy Hook Elementary School where he murdered 20 children and six adults.
"From my experience where you have the parent killed first, it's a fundamentally different circumstance," he told CBS News, "because the motivation for the homicide in large part arises from the relationship with the parent as opposed to anything else."
Mones is the author of "When A Child Kills," a book published in 1991 which he says proves that this problem has been around for a long time.
Each week in the United States, on average, about five parents are killed by their biological children (amounting to more than 250 cases annually). But Mones says that in only 15 percent of the cases does a boy kill his mother.
"I can't talk about this case (Newtown) because I don't know the details but I can tell you that in the dozens of other boys I've represented who have killed their mothers, that what happens is that they view the relationship as smothering," he said.
"The boys view that they'll never get away from the parent or that environment...that this person is not allowing them to do what they want to do with their lives."
The answer to why Adam Lanza committed this atrocity - if we ever will truly know - will be defined by Adam's relationship with this mother and what type of "family dysfunction" was present in the home, Mones says.
"There's only a couple of cases I can think about in the last 10 years I've done where there's been no family dysfunction...," he said. "Most of the circumstances you do find that and when you combine the mental health problems of a kid with some significant family dysfunction, you get these homicides."
It's not known what, if any, mental problems Adam Lanza had, although those who knew the family said his mother Nancy Lanza had told them that Adam had Asperberger's syndrome, which made it difficult for him to relate to others.
Very little is known about the relationship between mother and son except anecdotal reports that Nancy was demanding and taught Adam how to shoot guns, often bringing him to shooting ranges. Friends and neighbors have described her as generous, social and upbeat. She has also been described as a gun enthusiast who owned at least five guns.
"When I heard the mother owned the guns, it was no news to me," Mones said.
He said the more familiar a young person is with a gun and how to use it, the more that person becomes "desensitized" to its potential danger. "It's okay to have guns, they're not afraid to use the gun," Mones said. "The gun is something that becomes like furniture in the house."