Police in Northern California say a possible school shooting was prevented due to a tip line that led to the detainment of a student who shared questionable material online.

Police in Menlo-Atherton received a tip from its “Say Something” tip line on Sept. 10 that indicated a former student at an area high school had posted concerning content to Instagram referencing the high school.

Police locked down the Menlo-Atherton High School campus in addition to a nearby middle school for an hour until the student was located and weapons were recovered from the home. The investigation is ongoing, police say.

“Say Something” is an anonymous reporting system created by Sandy Hook Promise, a Connecticut-based organization that advocates for solutions to gun violence in schools. The organization said the Menlo-Atherton incident is the 19th “credible planned” school attack nationwide” the system has prevented since its launch in 2018. About 5,000 school districts partner with the organization and tips are accepted via a mobile app, text, phone, or through a website.

Jarrett Dooley, executive director of student services with the Sequoia Union High School District, which includes Menlo-Atherton school, said in a statement he is “grateful” that the tip line “resulted in swift action by local law enforcement and school administrators to keep all students, staff, and community members safe.”

Sandy Hook Promise CEO Nicole Hockley praised the student tipster for their actions.

“Sometimes we receive hundreds of tips about a potential threat. Other times, as we saw in California during this incident, it just takes one,” she said in a statement. “At a time where many feel powerless against gun violence, this student’s bravery proves that each of us has the ability to make a difference. Gun violence is not inevitable – it is preventable – and prevention begins when we recognize the signs and take action.”

Hockley was a mom of a victim at Sandy Hook.

Sandy Hook Promise says the top warning signs for potential violence at a school include bullying, withdrawing from friends or activities, chronic social isolation, direct threats against a place, a person or themselves, bragging about access to guns, obsession with online violent content, cruelty to animals, and recruiting others for an attack.

The incident took place on the same day as a mass shooting in Evergreen, Colorado, where a 16-year-old shooter shot two students and then himself. The two students were in critical condition afterward. The school reopened this week for classes.