FBI’s former top profiler warns of potential for ‘contagious’ copycat crimes in Nashville after school shooting

"In 2000 when the FBI released its first report on school shooters, we found that the copycat influence was powerful and it influenced the 18 cases that we studied," Mary Ellen O'Toole told the Daily Mail.

This report was compiled in the wake of the Columbine High School shooting in 1999, which claimed the lives of 13 people. O’Toole said that she believed the shooter’s manifesto should not be released in that case, over fears of the "contagion" effect.

Following such a shooting, O’Toole said that communities should remain on alert for up to two weeks due to the potential for copycats.

"The term now used is the 'contagion effect.' More shootings will follow the one in Tennessee. I am also expecting that there will be references to other shootings in the shooter's computer, her writings, her manifesto, etc," O’Toole, current director of the forensic sciences program at George Mason University, said.

"There will be evidence of 'injustice collecting' themes in those writings - which is a major motivator for many of these shootings," O'Toole continued.

In the 2000 report, O’Toole wrote, "School shootings and other violent incidents that receive intense media attention can generate threats or copycat violence elsewhere."

"Copycat behavior is very common, in fact. Anecdotal evidence strongly indicates that threats increase in schools nationwide after a shooting has occurred anywhere in the United States."

O’Toole urged school staff and police to "be more vigilant in noting disturbing students' behavior" following school shootings.

In regards to Columbine, O’Toole said, "I was brought into the Columbine case to review the basement tapes and provide an opinion as to whether or not those tapes should be released to the public."

"It was my opinion that they should not be released to the public - ever because it would influence other prospective shooters. They were never released but everything else about Columbine was."

Police have not yet revealed a motive for the shooting, but noted in a press conference that shooter Audrey Hale, who identified as transgender and was a former student of the school, had a detailed manifesto and a map of the school, which were discovered in the family home.

Three students were killed: Hallie Scruggs, the 9-year-old daughter of Chad Scruggs, the senior pastor at Covenant Presbyterian Church, Evelyn Cieckhaus, and William Kinney, both aged 9.

Custodian Mike Hill, 61, substitute teacher Cynthia Peak, 61, and school head Katherine Koonce, 60 were also killed in the shooting.

Surveillance video released on Monday evening showed Hale driving her Honda Fit to the Covenant School, and entering the building after shooting through the glass of the front doors.

Police said that Hale was equipped with at least two rifles and a handgun.