I don't remember it happening with Lanza, he had no social media presence whatsoever other than a couple YouTube accounts [one of which was only discovered very recently] but you're absolutely right with Nikolas Cruz. That fucking mongoloid had dozens and dozens of tips called in about him to local PD, Sheriff's office and the FBI, had a history of fighting, got in shit for bringing a knife to school, posted all kinds of red flag bullshit on social media [killing animals for no reason accompanied by photos of his guns] and also often harassed, stalked and threatened people he went to school with.
I know people always want to say: "Well detectives/the feds/etc can't be clairvoyant!" Of course they fucking can't, but they could take some time to do a cursory bit of investigative work can't they? As is their purvey and the entire reason they're being paid?
That much said, the guy in this case seems like your average garden variety BLM/wanna-be rapper sperg, and surprisingly he didn't post any photos of guns, drugs and greenbacks as many of his kinfolk like to do. I don't see anything too suspicious on his page. The posts about NFAC probably weren't enough to trip the radar in his case, I'm sure if it were you or me supporting a militia we'd be partyv& before the end of the business day.
At some point you have to accept that, in many cases [obviously not all] the feds know these guys are planning some heinous shit but neglect to act on that information, whether it's out of ineptitude, laziness, or because they want something to crowd up the news cycle to push another story out? That's up to you, I'd say a combo of all three. I don't necessarily think that's the case here, but anymore you have to look at such incidents with a reasonable suspicion that someone allowed it to happen. It's not like the feds are lacking resources, after all. They had plenty of resources and manpower to go after the boomers that took an impromptu tour of the Capitol building, and even used cell signals to find guys who just happened to pass by while the 'insurrection' was in progress.