NEW YORK -- A New York City high school student was arrested Thursday after bringing a loaded gun to class and writing on social media that he wanted to "shoot the school up," authorities said.

The 16-year-old sophomore posted the threat to Instagram around 10:15 a.m. while in class at Benjamin N. Cardozo High School in Bayside, Queens, Police Commissioner Jessica Tisch said. A person who saw the post alerted the FBI, which then contacted the NYPD, Tisch said.

Officers went to the school, located the student and found a 9mm handgun in his backpack that was loaded with 13 rounds of ammunition, Tisch said. Unlike some other New York City public schools, Cardozo High School does not screen students for weapons, she said.

"We are so grateful that this incident did not end tragically," Mayor Eric Adams said at a news conference announcing the arrest.

Police did not publicly identify the 16-year-old by name because he is a minor, Tisch said, adding that he had no prior criminal history. A message seeking information about the charges he faces was left for the Queens district attorney's office.

According to Tisch, the threatening post showed what appeared to be school work on a desk, and the caption included a slang term for anger or frustration and read, in part: "vow to shoot the school up." Location data showed that it was posted from the school, she said.

Police officers worked with school staff to locate the 16-year-old, removed him from his classroom without incident and took him to an empty conference room where they searched his backpack and found the gun, Tisch said. The boy also had three cell phones on him, she said, one in a school-issued locking pouch and two others.

Officers confirmed that the Instagram post came from the 16-year-old's account by calling a phone number associated with it, Tisch said. When officers called the number, the commissioner said, one of the boy's cellphones rang.

Adams, a former police captain, lamented the easy availability of guns, even as he touted the NYPD's success in making gun-related arrests and driving shootings, murders and other crime lower.

"This was a failure of a society that allowed a 16-year-old to get so close to shooting up a school and potentially killing classmates and teachers," Adams said. The boy, he said, "either would have ended up dead or in jail for many years" had he followed through on the threat.

Tisch said the swift law enforcement response and collaboration between the FBI, which received the tip, and the NYPD, which acted on it, was an example of the system working to prevent mayhem.

"I will be honest, as a mother of two young, school age boys, I am shaken," Tisch said. "But, as your police commissioner, I am resolute in ensuring that the NYPD does everything in our power to keep our children safe."

16-year-old with a loaded gun threatened to shoot up his NYC high school, police say