New disturbing details in the Jonathan Martin case ... we've learned he was arrested with a cache of weapons including a loaded shotgun, knife and an ax -- and had been threatening classmates for years. TMZ Sports has obtained court documents filed after Martin was taken into custody on Feb. 23 after posting a cryptic photo on Instagram threatening old high school classmates and former NFL teammates. Officials say they found Martin after getting a call about a "suicidal male with a gun" at a hospital in Glendale, CA. He was placed on a 72-hour psychiatric hold because officials believed he was a possible danger to himself and others. They searched his car and found a loaded 12-gauge shotgun, a large knife, an ax and a cell phone. A second shotgun was found at his parents' home.