Donald Trump has called the 19-year-old who carried out a deadly shooting at a Florida high school on Wednesday “mentally disturbed” and claimed that classmates knew he was “a big problem.” The president tweeted Thursday morning saying Nikolas Cruz—who was charged before dawn with 17 counts of pre-meditated murder—was expelled from his school for “bad and erratic behavior” and that people “must always report such instances to authorities.” Shortly after he took office, Trump rolled back an Obama-era rule that made it more difficult for people with mental-health issues to buy guns. Details are still emerging about the killer. The Miami Herald reported that Cruz was expelled after coming into school with bullets in his backpack, and a number of social-media posts attributed to the suspect have been seen as potential warning signs that he was planning a mass shooting. The Fort Lauderdale Sun-Sentinel reported that Cruz’s adopted mother died last November, and that Cruz had since been living with family friends who made him keep his firearms in a locked cabinet, for which he had the key.