At least 17 people were killed Wednesday when a gunman opened fire at Stoneman Douglas High School in Parkland, Florida, multiple outlets including CNN and ABC reported. That makes it the deadliest school shooting in the U.S. since the 2012 Sandy Hook massacre that killed 26 people in Newtown, Connecticut. According to law-enforcement officials, the alleged shooter, identified as Nikolas de Jesus Cruz, 19, is a former student of the South Florida high school, and one math teacher identified him as having been perceived as a threat in the past. “We were told last year that he wasn’t allowed on campus with a backpack on him,” Jim Gard, the teacher, told the Miami Herald. “There were problems with him last year threatening students, and I guess he was asked to leave campus.” Cruz reportedly used an AR-15 rifle during his shooting spree—the same weapon used by Adam Lanza during the Sandy Hook massacre.