YouTube vlogger Ben Bennight contacted the FBI in September after a user named Nikolas Cruz commented, "I'm going to be a professional school shooter," on one of his videos, according to BuzzFeed News. The FBI interviewed Bennight the following day to ask if he knew anything about Cruz, and contacted him again Wednesday after the 19-year old allegedly killed at least 17 people at Marjory Stoneman Douglas High School in Parkland, Fla.

Why it matters: Details about Cruz's background and disturbing social media presence continue to emerge, painting a picture of a troubled teenager who struggled with mental health and disciplinary problems.