Thoughts On Adam Lanza

The FBI has "unsealed" some documents about alleged Sandy Hook shooter Adam Lanza. You can read about them (but not see the actual documents) here:

http://www.cnn.com/2017/10/24/us/sandy-hook-adam-lanza-unsealed-docs/index.html

If any of this is true, I have two questions:

1. Why wasn't this reported to the authorities BEFORE the alleged shooting; and
2. Why didn't the FBI stop him?

Don't they monitor any "threats" other than Republican political candidates, their families and business associates?