Mourners stand during a candlelight vigil for the victims of the shooting on February 15

A troubled teenager has confessed to gunning down 17 people at his former high school in Florida, court documents showed, as the FBI admitted it had received a tip-off about the 19-year-old gunman yet failed to stop him. Terrified students hid in closets and under desks on Wednesday at Marjory Stoneman Douglas High School in Parkland, texting for help as the gunman, Nikolas Cruz, stalked the school with a semi-automatic AR-15 rifle. Cruz 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.