On a day when tens of thousands of students across the county walked out of classes to protest gun violence, the sister of notorious mass killer Dylann Roof was arrested in South Carolina for carrying a knife and pepper spray on school property.

Morgan Roof, 18, a student at A.C. Flora High School in Columbia, was charged Wednesday with two counts of carrying weapons on school property and possession of marijuana, according to records at the Alvin S. Glenn Detention Center where she was taken.

Students at the school became alarmed Wednesday at her Snapchat post disparaging National Walkout Day, which was being held in response to a mass shooting at a high school in Parkland, Fla., that left 17 dead. Roof’s post said she hoped “it’s a trap and y’all get shot” and “we know it’s fixing to be nothing but black people walkin out anyway,” authorities told local news outlets.

The Richland County Sheriff’s Department confirmed to The State newspaper that Morgan is sister of Dylann Roof, who is on death row at a federal prison in Indiana for fatally shooting nine black members of a Charleston church during a Bible study in 2015. Roof, a self-described white supremacist told authorities that he targeted a historic black church in hopes of starting a race war. A month after the killings, South Carolina removed the Confederate flag from the statehouse grounds, ending its 54-year presence at the Capital.

[ Dylann Roof’s racist manifesto: ‘I have no choice’ ]

In December 2016, a jury convicted Roof on 33 federal hate crime charges in connection with the killings at Emanuel African Methodist Episcopal Church. He was sentenced to death in January 2017. Four months later, he pleaded guilty to state charges — nine counts of murder, three counts of attempted murder charges and a related weapons charge. A deal with prosecutors allowed him to avoid a second death penalty trial and be given a sentence of life in prison without parole.

After Wednesday’s incident, Morgan Roof is not allowed to return to school, WLTX-19 reported.

“A student used social media to post hateful messages,” Susan Childs, the school’s principal, said Wednesday in a statement to parents. “The posting was not a threat, but was extremely inappropriate. That student was dealt with in a swift and severe manner as the posting caused quite a disruption.”

A second student was also arrested at the school this week for allegedly having a loaded magazine, The State reported. A Smith Wesson .380 handgun was recovered on school grounds with information provided by students, authorities told the news outlet. The student, who was not identified, was released to the custody of his parents, the newspaper said.

The principal’s statement did not mention the arrests. On the second matter, she wrote: “In an isolated incident yesterday, administration was notified that there was possibly a weapon on campus. Through diligent work from the Assistant Principals and the School Resource Officers, an unloaded weapon was recovered. The investigation of this matter involves law enforcement as well as school officials. ”

Gov. Henry McMaster said on Twitter that “we owe a debt of gratitude to all involved” for acting quickly and decisively.

“Potential tragedy was avoided at AC Flora High School. In two separate incidents, students and educators reacted quickly to reports of suspicious activity and behavior to their Richland County Sheriff’s Department school resource officer.”

Potential tragedy was avoided at AC Flora High School. In two separate incidents, students and educators reacted quickly to reports of suspicious activity and behavior to their Richland County Sheriff’s Department school resource officer. (1/)

— Henry McMaster (@henrymcmaster) March 15, 2018

More from Morning Mix:

Gun-trained teacher accidentally discharges firearm in Calif. classroom, injuring student