Nikolas Cruz, described as a loner by a former classmate, had once been expelled from a Parkland, Florida, high school for disciplinary reasons.
On Wednesday, the 19-year-old former student returned to Marjory Stoneman Douglas High School and is suspected of gunning down at least 17 people, in a massacre that has now become one of the 10 deadliest mass shootings in modern US history.
Broward County Sheriff Scott Israel said Cruz's digital profile contained what he called "very, very disturbing" content.
He had a variety of gun and violence-related postings on social media sites.