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Former NFL lineman Jonathan Martin, who five years ago accused Miami Dolphins teammates of bullying him, was detained by authorities Friday after an Instagram post appeared to threaten his former high school — which cancelled classes as a precaution.

Martin was not arrested, law enforcement sources familiar with the investigation told NBC News.

Instead, on Friday he was at a medical facility in the Los Angeles area seeking treatment. The sources would not elaborate on the treatment or the circumstances that led to it.

His alma mater, Harvard-Westlake School in Los Angeles, closed for the day "due to a security threat," according to a message sent to parents. The alert did not say anything more about the threat, but said the school would release more information later in the day.

Martin went public with his allegations of bullying in 2013, in the middle of his second year in the NFL, saying he was harassed by fellow offensive lineman Richie Incognito and others.

The Dolphins traded him to the San Francisco 49ers in 2014. The 49ers released him in 2015, and he was picked up by the Carolina Panthers, but never played for them. He retired that year, citing a back injury.