According to TMZ Sports, Martin will be charged with four felony counts of making threats and one misdemeanor count of carrying a loaded firearm in a vehicle.
The charges stem from a threatening post on Martin's Instagram that surfaced Feb. 22.
According to NFL.com's Austin Knoblauch, Martin's post showed a shotgun with ammo and included the caption: "When you're a bully victim & a coward, your options are suicide, or revenge."
Martin tagged former Miami Dolphins teammates Mike Pouncey and Richie Incognito in the post, as well as Harvard-Westlake, which is where he attended high school.
WSVN 7 News relayed a screenshot:
Harvard-Westlake subsequently closed for the day on Feb. 23 as a precaution. Citing court records, USA Today's A.J. Perez reported the school later sought a restraining order against the 28-year-old.
A 2012 second-round pick of the Dolphins, Martin spent two years in South Florida before making a move to the San Francisco 49ers.
His time with the Dolphins ended in controversy after he accused Incognito of bullying. An investigation facilitated by the NFL and led by Ted Wells later found a "pattern of harassment" captained by Pouncey, Incognito and offensive lineman John Jerry, according to Sports Illustrated's Jenny Vrentas.