"The individual we believe responsible for the social media post in question has been detained and our investigation is ongoing," the LAPD said in a statement.
Hours later, the LAPD said the "person of interest" in the case had been questioned and was no longer in custody.
It isn’t clear if Martin posted the image or he was hacked, law enforcement sources said.
The post was found Thursday evening, and on Friday morning, the school sent out an emergency notification asking students to stay home.
“Students already on campus will be supervised and protected in designated areas until parents can make arrangements to pick them up or for them to return home safely,” the notification said.
In a statement, the school said: "Late last evening, Harvard-Westlake was made aware of a disturbing and possibly threatening social media post attributed to a former student, which specifically mentioned our school. The safety of our students, faculty, and staff is always our top priority, so we made the decision to close both campuses of our school today. It is our understanding that the former student is now in the custody of authorities. We are working closely with law enforcement, and all further inquiries about this matter should be directed to the Los Angeles Police Department."
Martin hasn’t played in the NFL since 2015. Shortly after his retirement, he wrote a lengthy Facebook post sharing that he had suffered from depression and had contemplated committing suicide.