The FBI has launched a review about its handling of a warning about the teenager accused of carrying out Wednesday’s school shooting in Florida. Nikolas Cruz, 19, has appeared in court charged with murder. With 17 people killed, this is the deadliest US school shooting since 2012.

Cruz, who had links with a white supremacist group, reportedly commented on YouTube last year that he would be a “professional school shooter.” A user alerted authorities to the post.

Teachers were also warned about Cruz, who was not allowed on campus with a backpack, US media report. The FBI said it had investigated the YouTube comment but had been unable to fully identify the person who posted it.

The attack happened at Marjory Stoneman Douglas High School in Parkland, Florida. Fourteen people were wounded, three of whom remain in a critical condition. On Thursday, Cruz confessed to investigators that he carried out the shooting, adding that he had a backpack full of ammunition.