In the case of the YouTube alert that the FBI took...I would imagine it was made into a report and sent into some massive database. There...it sat.
Go ask the FBI...on an average day, they probably get 10,000 tips. They aren’t capable of correlating this into any kind of useful information.
Even if the YouTube guy had gone to the local cops in the city where this kid lived...other than a visit by the cops, I doubt if anyone would have done anything much.
And their response after the shooting was pure idiocy. They couldn't trace the poster's name. I guess they never heard of tracing an IP address? And these are the wombats who are supposed to be protecting us from terrorism?
They could have checked his name against the background check data base.