LAKE WORTH BEACH, Fla. – A recent graduate of Lake Worth Community High School was arrested Monday after he posted a threatening Instagram message directed at his alma mater, deputies said.

Antonio Ruiz, 19, faces a felony charge of sending a written threat to kill, do bodily injury or conduct a mass shooting or an act of terrorism.

According to a Palm Beach County Sheriff's Office probable cause affidavit, Ruiz posted a picture of Lake Worth Community High School along with the caption, "Hope your book bags are bulletproof," on Instagram.

Deputies said surveillance video showed that Ruiz had been on campus that same day.

According to the affidavit, Ruiz confessed to posting the picture and caption but said he was "only playing a prank." Ruiz, who graduated from the school last year, said he was there with his girlfriend, who needed to get some school transcripts.

Ruiz was being held in the main Palm Beach County jail on a $5,000 bond.