With age and bitter experience come knowledge and wisdom. Having dealt with government employees all my life, I know exactly what to expect. Some want to get things done. Most are only in it for the check, especially after a few disillusioning years on the job.
Why do you think so many mass shooters turned out to be people law enforcement already knew about? Answer: because law enforcement is not highly motivated to take preventive measures. Not when there is significant work involved. They're okay at responding to crimes in progress, but they underperform in other areas. If you tell the cops your neighbor is in your house with a samurai sword, they will come right over. If you tell them he waves a gun at you and threatens to kill your family once a week, and you have no witnesses and no video, you are on your own.
Look at it this way:
Scenario A: FBI (or local cops) respond to OP's warning. They investigate. They write reports. They consult prosecutors to see what the best course of action is. They decide on a way to approach the nut and get his firearms. Then there is a shootout, or the nut complies and then sues them. Either way, their quiet lives have been disrupted, and they have been put to a great deal of grief. Their reputations and pensions, and the careers of the attorneys (very ambitious people who don't want to be underpaid prosecutors all their lives) have been put at risk. They might have to get real, private sector jobs where it's much easier to get fired and there are no pensions.
Scenario B: FBI brushes OP off. They do nothing whatsoever. Then one of two things happens:
1) the nut never harms anyone, and the government employees are in the clear. They took no risks and did no work. Their reputation is safe, and so are their pensions; or
2) the nut shoots up a school or mall. About 1% of the populace criticizes the government for a while. A week later, they've stopped complaining. The government employees are in the clear. They took no risks and did no work. Their reputation is safe, and so are their pensions.
Alternative 2 happened with Nikolas Cruz. Except for one LEO who hid while the kids were shot (and got caught on camera), none of the many cops who dealt with Cruz in were held accountable. And Cruz was one of their best customers. He was notorious.
People who know how the world works know which course law enforcement or any other government entity will usually take, and they know they will get pushback for talking about it.