It appears that Marjory Stoneman Douglas HS shooter Nikolas Cruz was known to the FBI as a threat back in September 2017 thanks to a YouTuber who alerted law enforcement over a comment left on a video:

The FBI was Warned About A School Shooting Threat From Nikolas Cruz In September https://t.co/wqwMlqq5JC

From BuzzFeed:

Last fall, a Mississippi bail bondsman and frequent YouTube vlogger noticed an alarming comment left on one of his videos. “I’m going to be a professional school shooter,” said a user named Nikolas Cruz.

The YouTuber, 36-year-old Ben Bennight, alerted the FBI, emailing a screenshot of the comment to the bureau’s tips account. He also flagged the comment to YouTube, which removed it from the video.

Agents with the bureau’s Mississippi field office got back to him “immediately,” Bennight said, and conducted an in-person interview the following day, on Sept. 25.

“They came to my office the next morning and asked me if I knew anything about the person,” Bennight told BuzzFeed News. “I didn’t. They took a copy of the screenshot and that was the last I heard from them.”

FBI agents contacted Bennight again Wednesday, after a 19-year-old named Nikolas Cruz allegedly opened fire at a Marjory Stoneman Douglas High School in south Florida, killing at least 17 people.