Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

The 18-year-old sister of convicted Charleston church shooter Dylann Roof was arrested Wednesday at her high school following a social media post that "caused alarm" to other students, authorities said.

A school resource officer at A.C. Flora High School in Columbia, South Carolina, allegedly discovered marijuana, pepper spray and a knife in Morgan Roof's possession.

No one was harmed in the incident, Richland County Sheriff Leon Lott said in a statement.

The Morning Rundown Get a head start on the morning's top stories. This site is protected by recaptcha

Morgan Roof was charged with simple possession of marijuana and two counts of carrying weapons on school grounds.

She had reportedly earlier posted a message on Snapchat referring to Wednesday's national school walkout against gun violence that also honored the 17 people fatally shot last month at a Parkland, Florida, high school.

The purported post read that "your [sic] walking out for the allowed time of 17min, they are letting you do this, nothing is gonna change what tf 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," NBC affiliate WIS reported.

NBC News could not immediately verify that Morgan Roof posted the message.

In a letter to parents and students, A.C. Flora Principal Susan Childs said that the walkout at the school generally went well, but referenced an incident in which a student "used social media to post a hateful message."

"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," Childs said.

Morgan Roof was granted a $5,000 bond and ordered not to attend school while out, WIS reported.

Her brother, Dylann Roof, is awaiting execution at the Terre Haute Federal Prison in Indiana after he was sentenced to death in January 2017. The self-proclaimed white supremacist killed nine black worshippers at a Charleston church in 2015.