Authorities have allegedly stopped a chilling plot to carry out a “mass casualty attack” at a Houston-area high school, arresting two teenage girls in connection with the scheme.

The FBI Houston Branch first posted an update online about the arrests on Wednesday, saying, “FBI Houston received information yesterday that two underage teenagers from the Houston area were plotting a mass casualty attack at a local school. Our Threat Mitigation Team immediately responded to help identify and arrest them.”

This situation is why FBI Houston exists: to protect our community & assist our partners. We received troubling information, conducted a rapid investigation, shared findings with LE partners, & ultimately saved lives,” The Bureau added.

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The plot from the teens, which reportedly involved pipe bombs and a shooting attack at Memorial High School in Houston, was uncovered after authorities discovered detailed plans shared through social media, reported CBS News.

According to the Spring Branch Independent School District police, the suspects, females aged between 16 and 15, were arrested following an investigation into a series of online threats.

The Montgomery County Sheriff’s Office (MCSO), located just north of Houston, confirmed the arrests and revealed that one of the suspects is a student at Willis High School, while the other is enrolled in a school within Spring Branch ISD. Although their names have not been released due to their age, one suspect has been charged with making a terroristic threat, per KHOU 11.

Investigators believe that the plot was credible due to the detailed nature of the plans, which included specific information on how the attack would unfold.

In response, Spring Branch ISD sent a letter to Memorial High School students, families, and staff, informing them of the situation and reassuring them that the police had already intervened.

The MCSO office relayed that the investigation is ongoing, and they are “treating this matter with the utmost urgency and commitment, as threats of violence in our schools will not be tolerated.”