Harvard-Westlake also has a second middle school campus in Holmby Hills, which was expected to remain open Friday.
One source at Harvard-Westlake said Martin has been detained over the incident, though police said there was "no credible threat" against students.
This frightening event comes on the heels of the February 14 massacre at Parkland's Marjory Stoneman Douglas High when 17 students and staffers were gunned down inside the school by a former student.
"We have confirmed that he's been located - the suspect, in this case", Sgt.
It's unclear if Martin posted the picture.
A source who had access to the picture said: "This seems like a joke gone horribly awful wrong".
Democrat flips Kentucky House seat in Republican territory
Belcher previously held the seat from 2008 to 2012 and from 2014 to 2016, when she lost to Dan Johnson, Thehill.com reported. Dan Johnson killed himself in December after adamantly denying the sexual assault allegations from the pulpit of his church.
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. In a May 2016 interview, Martin said there would never be any reconciliation with Incognito.
He ultimately quit the team and debated giving up on his career, feeling such psychological duress that he said he twice considered committing suicide.
Incognito was suspended for bullying Martin, other teammates and a team employee.
Martin played in three National Football League seasons, spending 2012 and 2013 with the Dolphins and 2014 with the San Francisco 49ers.
A patrol vehicle is seen at Harvard-Westlake's Studio City campus after classes were canceled on February 23, 2018.
An Instagram post of a gun that mentioned the school led to the campus being closed as a precaution on Friday, Feb. 23, 2018.
Law enforcement tells TMZ Sports.