"I'm going to be a professional school attacker."
Those are the words that allegedly wrote on the Internet Nikolas Cruz, the 19-year-old who opened fire on Wednesday afternoon at Marjory Stoneman Douglas High School in Parkland, Florida.
The shooting left at least 17 dead and reopened the debate in the United States on how to prevent massive attacks and how to detect potential threats.
In the hours after the attack, authorities recognized that Cruz had shown symptoms of being an unstable youth and that even the FBI had been alerted to his behavior.
On Thursday, US President Donald Trump said the attacker had left "many signs" in the past that showed he was "mentally disturbed," and that neighbors and friends "knew he was a big problem."