Two teenage girls in the US state of Texas have been arrested for plotting a ?mass casualty attack? with pipe bombs and guns, police say.

The girls, ages 15 and 16, were planning an attack at Memorial High School in a suburb of Houston, police told KHOU, an affiliate of the BBC?s US partner CBS News.

The plot - which was shared on social media ? was detailed enough that officials deemed it to be credible, CBS reported.

The Federal Bureau of Investigation (FBI) said its Threat Mitigation Team was involved in the response to help local police arrest the teenagers immediately.

?We are treating this matter with the utmost urgency and commitment, as threats of violence in our schools will not be tolerated,? the Montgomery County Sheriff?s Office said in a statement.

A letter sent to Memorial High School families said police quickly arrested a student over a ?potentially credible threat in the planning stages identified on social media?.

The school said it was implementing additional safety measures after the incident.

The sheriff?s office said one of the teenagers, a student at Willis High School in Conroe, Texas, was being held on unrelated charges, while the other, a student in the Spring Branch school district nearby, was in custody in a nearby county. Police have not named the two girls, as they are not adults.

The students were arrested within 30 minutes of the FBI calling to notify the school about the threat, according to ABC News. (BBC)

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