Two teenage girls accused of planning a “mass casualty attack” at a Houston-area high school were arrested, authorities said.

The girls, 15 and 16, were taken into custody on Tuesday. Their names were not released by authorities because they are minors.

The FBI field office in Houston said it had received information “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 @houstonpolice & @MCTXSheriff identify and arrest them,” the agency said in a post on X.

The alleged threat involved the girls using guns and placing pipe bombs at Memorial High School in the Houston suburb of Hedwig Village, NBC affiliate KPRC of Houston reported. Authorities said the girls were in the early stages of planning the attack and were arrested within 30 minutes of learning about it, the station reported.

Spring Branch ISD Police Chief Larry Baimbridge told KPRC that they were informed about the threat from a source.

“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,” FBI Houston said.

The 16-year-old girl is a former student at Memorial High School but was currently attending an alternative school in the district, according to KPRC. The 15-year-old is a Willis High School student.

Willis Independent School District said it was made aware of the incident on Wednesday.

“We are so grateful to the FBI and other law enforcement agencies that work together to keep students across the nation safe from those who intend to harm others,” the district said in a statement. “This incident affirms the importance of awareness and vigilance in protecting our students and staff.”

The Montgomery County Sheriff’s Office said additional security measures were implemented at Memorial.

“We are treating this matter with the utmost urgency and commitment, as threats of violence in our schools will not be tolerated,” the sheriff’s office said in a news release.

The 16-year-old was charged with making a terroristic threat, a third-degree felony, KPRC reported. The 15-year-old was in custody on unrelated charges.