As one southern California high school reopened Thursday after a two-day lockdown following threats from an anonymous social networking app, another was put on lockdown.

In both instances, the threats came from an app called Yik Yak, which allows users to post anonymously from anywhere within a 1.5 mile radius of their location.

The first threat was aimed at Mira Costa High School, in Manhattan Beach, an affluent coastal city near Los Angeles, on Monday and prompted the school to cancel classes until Thursday, according to Reuters. Torrey Pines High School in San Diego was put on lockdown Thursday morning after school officials were made aware of similar threats made on the app, NBC reported.

Police told Reuters that the post that prompted the lockdown at Mira Costa High read, "If you go to Costa you should watch out very closely at school today." The threat was reportedly followed by others throughout the day, including one that read, "Nice try costa, today was just a drill."

The school was re-opened Thursday with heightened security on campus.

San Diego police responded to the online threat at Torrey Pines High around 10:25 a.m. Thursday and by 11:30 a.m., parents had reportedly lined up outside the school as police searched the rooms before classes were dismissed shortly after, according to NBC.

Threats of mass shootings, bombings or other violence made through Yik Yak have have become a growing problem for schools across the country. And with mass school shootings at Marysville-Pilchuck High and Florida State University within the past month, school officials seem to be on high alert. Two students were suspended from a Des Moines-area high school Thursday after police determined they were responsible for posting a series of threats about a mass shooting at their school. At University of North Carolina, Chapel Hill students were notified Thursday morning about a bomb threat targeting the campus that was made via Yik Yak.