The perceived threat, posted on social media, is a Snapchat photo that shows boxes of ammunition with a caption that reads, "getting ready for graduation."
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According to the Orange County Sheriff's Department, the photo was posted a month ago and resurfaced this week.
The Capistrano Unified School District sent a message to staff, parents and students of the Rancho Santa Margarita School, alerting them, but saying they do not consider the threat to be credible.
"Our protocol with anything that even resembles a threat is to contact the Sheriff's Department...The Sheriffs have found no credible threat to our graduation," the message read.
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Sheriff's officials also released a statement saying the post was deemed not credible and did not rise to the level of a threat. They added that no one was reprimanded and no crime was committed.
Nevertheless, school officials asked students to arrive 30 minutes early to the graduation ceremony, which is being held at Orange Coast College, so that they can go through a metal detector.
District officials said the graduation will have the metal detectors "in light of what's happened in the country this year."