According to ESPN, Martin was detained because of the post.
In this September 30, 2013 file photo, Miami Dolphins guard Richie Incognito (68), center left, and and tackle Jonathan Martin (71), center right, sit on the bench in the second half of an NFL football game against the New Orleans Saints in New Orleans.
In addition, Martin also tagged the Instagram accounts of Richie Incognito and Mike Pouncey, two of the former lineman's ex-Dolphins teammates.
The image also had hashtags #HarvardWestlake - his high school - and #MiamiDolphins on the rifle. It reads, "When you're a bully victim & a coward, your options are suicide, or revenge".More news: Berkshire Hathaway says tax law gives it $29B boost in 4Q
Harvard Westlake shut classes down on Friday amid the threat.
Former Miami Dolphins lineman Jonathan Martin was detained by police after a threatening social media post forced his alma mater high school to close down Friday afternoon, multiple outlets reported.
"The safety of our students, faculty and staff is always our primary concern", Commons said in the email.
Martin was in southern California when he was detained pending further investigation by the Los Angeles Police Department's Major Crimes Division. "With these precautions in place, we believe there is no imminent threat to our campuses or our school community".More news: Bruce McArthur now charged with murders of six men: Toronto police
Representatives for the Miami Dolphins and the National Football League declined to comment. An NFL investigation found that Incognito, Pouncey and teammate John Jerry engaged in persistent harassment directed at Martin. He played 15 games for the San Francisco 49ers the following season, starting seven of them, and was claimed off waivers by the Carolina Panthers in March 2015. However, he retired before playing in 2015 after suffering a back injury. Martin has since said in interviews that he suffers from depression and has attempted suicide in the past.
PALO ALTO, CA - NOVEMBER 27: Jonathan Martin #55 and Richard Sherman #9 of the Stanford Cardinal celebrate after they beat the Oregon State Beavers at Stanford Stadium on November 27, 2010 in Palo Alto, California. You either sleep 12, 14, 16, hours a day when you can, or not at all.
A law enforcement source told KABC-TV in Los Angeles that Martin is now being held in a mental health facility. He detailed his struggle in a post on Facebook, saying he hoped it might help other struggling kids "know that they aren't alone".More news: Former Under Armour president accused of sexual misconduct