Most students who committed deadly school shootings over the past decade were either badly bullied, had a history of disciplinary trouble, or had previously behavioral concerns that were unreported, according to a U.S. Secret Service study released Thursday. The report said that in at least four cases, the gunmen wanted to emulate other school shootings, including the attacks at Columbine High School in Colorado, Virginia Tech University, and Sandy Hook Elementary School.

“These are not sudden, impulsive acts where a student suddenly gets disgruntled,” Lina Alathari, the center’s head, said in an Associated Press interview. “The majority of these incidents are preventable.” The report documents 41 school attacks from 2008 through 2017 at K-12 schools—all involving an attacker who was a student who injured or killed at least one person at the school while targeting others.