Nikolas Cruz has confessed to gunning down 17 people at his former high school in Florida, court documents showed Thursday, as the FBI admitted it had received a tip-off about the troubled teen yet failed to stop him. As Americans reeled from the countrys worst school massacre since the horror at Sandy Hook six years ago, President Donald Trump suggested the root cause of the violence was a crisis of mental health -- and defied calls to address gun control. Terrified students hid in closets and under desks on Wednesday at Marjory Stoneman Douglas High School in Parkland, texting for help as Cruz, 19, stalked the school with a semi-automatic AR-15 rifle. The gunman has been charged with 17 counts of premeditated murder, appearing Thursday afternoon via video link before a judge who ordered him held without bond. More than a dozen other people were injured in the shooting spree. "Today is a day of healing. Today is a day of mourning," Broward County Sheriff Scott Israel said. After being read his legal rights, "Cruz stated that he was the gunman who entered the school campus armed with a AR-15 and began shooting students that he saw in the hallways and on the school grounds," court documents showed. After the shooting, he stopped at a Wal-Mart and then McDonalds, Israel told reporters. He was detained 40 minutes later, after police identified him using school security camera footage. In a somber televised address to the nation in response to the 18th school shooting so far this year, Trump vowed to make mental health a priority -- after tweeting about the "many signs" the gunman was "mentally disturbed" -- while avoiding any talk of gun curbs.  A few US states have laws allowing police and family members to obtain orders barring people suspected of being a threat from possessing guns, but Florida does not. Some gun control proponents and legal experts said Wednesdays shooting might have been thwarted if it had. Cruzs history of violence in school and disturbing social media posts would have allowed authorities to prevent him from legally obtaining a firearm in California and Connecticut, which have gun violence restraining order (GVRO) statutes, the experts said. US authorities are under scrutiny after the FBI confirmed it was alerted last September to a message posted on YouTube, in which a user named Nikolas Cruz vowed: "Im going to be a professional school shooter." In a statement, the FBI said it had carried out "database reviews and other checks" but was unable to identify the person who made the post.