Morgan Roof, sister of avowed white supremacist Dylann Roof, is shown after being arrested for carrying weapons at a high school, in this police photo released in South Carolina, U.S., on March 15, 2018.
Morgan Roof, sister of avowed white supremacist Dylann Roof, is shown after being arrested for carrying weapons at a high school, in this police photo released in South Carolina, U.S., on March 15, 2018. Courtesy Richland County Government/Handout via Reuters

The sister of convicted American mass murderer Dylann Roof was arrested on Wednesday on drug and weapons charges. Morgan Roof, 18, posted a disturbing message on Snapchat, calling for the death via gunshot of the thousands of students who walked out of class and protested gun violence.

Morgan brought a knife, pepper spray and cannabis at her school, A.C. Flora High School, in Columbia, South Carolina on Wednesday. She was charged with simple possession of marijuana and two felony counts of carrying weapons on school grounds.

Morgan was taken to the Alvin S Glenn Detention Center. She has since been released on a US$615 (AU$796) cash bond and will face court at a later date.

Her arrest came as thousands of students in the US walked out of their classrooms and joined the protest against gun violence on Wednesday. She posted on the social media app Snapchat that she hoped the students participating in the national walkout would be shot. The post alarmed her fellow students, but it was the stuff that she brought to the school that got her arrested.

“Your walking out for the allowed time of 17, they are letting you do this, nothing is going to change what the f--- you think it’s gonna do? I hope it’s a trap and y’all get shot. We know it’s fixing to be nothing but black people walkin out anyway,” [sic] she wrote.

She later told an investigator that she brought the weapons with her “for protection,” Lt Curtis Wilson, spokesman for the sheriff’s department, was quoted by The State as saying.

Dylann Roof, Charleston church shooter

Morgan is the younger sister of Dylann, the white supremacist mass murderer who fatally shot nine African Americans and injured one other person at the Emanuel African Methodist Episcopal Church on June 17, 2015. He admitted that he committed the shooting in the hopes of starting a race war.

Dylann, who will turn 23 in April, received 33 federal hate crime charges and 13 state charges. He was sentenced to those for the federal charges, as well as nine consecutive sentences of life without parole for the state murder charges. He is currently at the USP Terre Haute in the state of Indiana as a death row inmate.

Dylann and Morgan share the same father, Franklin “Benn” Roof, but have different mothers. Paige Roof is Morgan’s mother and Dylann’s stepmother. His mother, Amy Cowles, was divorced from Benn. Dylann also has an older half-sister.