Ex-NFL player Jonathan Martin -- who was at the center of the Miami Dolphins bullying scandal -- has been detained by police ... after he allegedly posted a disturbing photo of a gun, ammo and the names of ex-NFL teammates.
The post first appeared on Martin's Instagram account -- showing an image of a shotgun surrounded by ammunition with the caption, "When you're a bully victim & a coward your options are suicide, or revenge."
He also listed social media handles for ex-Dolphins teammates Richie Incognito and Mike Pouncey.