After further review of Christopher Harper-Mercer, the gunman who opened fire and killed nine people at Oregon community college, evidence shows that his mother, Laurel Harper, was an avid gun user.

The Oregon community college and Sandy Hook elementary school shootings both follow this similar patterns, according to CNN. Both shootings had murderers who shared a bond of firearms with their mothers. The two shooters, Harper-Mercer and Adam Lanza, had social issues and mental health problems. 

"When you begin to bring guns into the home environment where you have that dangerous cocktail of behavior, that's pretty unbelievable," said Mary Ellen O'Toole, a former FBI profiler who directs George Mason University's forensic science program, The New York Times reported. The two gunmen had mothers that are pro gun and helped to arm them with a large supply of firearms. The two mothers even took Mercer and Lanza to shooting ranges to practice proper gun use.

"I keep two full mags in my Glock case. And the ARs & AKs all have loaded mags. No one will be 'dropping' by my house uninvited without acknowledgement," Harper's mother wrote on an online forum obtained by The New York Times.

Harper-Mercer's father, Ian Mercer, did not know that his son had guns, according to ABC 7.  "How on earth could he compile 13 guns? How could that happen?" Ian Mercer told CNN on Saturday.

"In some cases, (parents) don't recognize there's a problem," said Peter Langman, psychologist and author of two books on school shooters. "In other cases, they're aware of their child's mental health issues, but they don't see any evidence of violence, so they don't see any reason not to take their kid target shooting." Langman added that parents might use guns as a way to bond with children who are socially isolated due to mental health issues.