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Former NFL offensive lineman Jonathan Martin has reportedly been taken into custody after a post on his Instagram page led to his former high school in California being shut down, per Matt Stone of ABC News.

According to James Queally, Richard Winton and Eric Sondheimer of the Los Angeles Times, Harvard-Westlake High School closed Friday morning in the wake of a "security risk" due to the disturbing post from Martin's Instagram account, though it's not certain if Martin is the one who made the post.

Nick Brown of 12Up.com captured a screenshot of the image on Martin's official Instagram page featuring a gun:

"The individual we believe responsible for the social media post in question has been detained and our investigation is ongoing," the Los Angeles Police Department said, per Kyle Bonagura of ESPN. LAPD did not confirm reports that it is Martin who is in custody.

TMZ reported police are treating the post as a "serious threat."

"Last evening, we learned of an Internet post that mentions Harvard-Westlake by name," the school said in a statement, via Justin Terranova of the New York Post. "Out of an abundance of caution, and because the safety of our students, faculty, and staff is our top priority, we made the decision to close school today. We are working closely with law enforcement and will share more information when we are able."

Martin attended Harvard-Westlake High School, located in Los Angeles, prior to playing college football at Stanford.

The Instagram post on Martin's page also mentions Richie Incognito and Mike Pouncey, who played alongside him on the Miami Dolphins' offensive line in 2012-13.

Ted Wells, an independent investigator sanctioned by the NFL, found that Incognito, Pouncey and John Jerry engaged in a "pattern of harrassment" toward Martin.

Last week, per CNN.com, Nikolas Cruz confessed to killing 17 people at Marjory Stoneman Douglas High School in Florida.