The school shooting in Oregon last week has similarities to a Connecticut elementary school massacre three years ago.
Both killers had mental health issues, an interest in mass murders and were loners living with protective mothers in houses full of guns.
Former FBI profiler Mary Ellen O'Toole says Christopher Harper-Mercer fit the profile of many mass killers.
O'Toole says Harper-Mercer was a calm young man who lacked empathy as he slaughtered students at the rural Oregon community college.
"Profiles of mass shooters have determined that by and large they are young men who are very isolated, loners, socially awkward, angry, bearing grievances that really leads them to seek revenge, retribution...a sense of empowerment through these mass shootings," said Dr. Hank Schwartz, Hartford Hospital's psychiatrist-in-chief.
As a national expert and a member of the governor-appointed Sandy Hook Commission, he said both Harper-Mercer was similar to Connecticut killer Adam Lanza because of their interest in guns.
Both of their moms reportedly took them to shooting ranges.
"In both instances, the parents really colluded with their kids to an extreme degree and we know gun violence and the presence of assault weapons is the single biggest factor in mass shootings," Schwartz said.
But there is no indication either expected their children to become violent.
"The single biggest obstacle to determining the motive is that most of these cases the shooters are killed so we can't speak with them. We can't ever really know what was on their minds," Schwartz said.
Copyright 2015 WFSB (Meredith Corporation). The Associated Press contributed to this report. All rights reserved.