“The Family Warned Local and Federal Authorities About HIs ‘potential for violence”” is basically a meaningless statement
How many angry spouses falsely accuse their partners of harboring violent tendencies? How many people hate their bosses or co-workers? How many people are angry at their parents?
Are cops and FBI supposed to hunt down and detain everyone who has a complaint against them? And then what?
Well, a lot of spouses do that...and a lot of orders of protection are filed...every day.
There are still too many details we don’t have to make any judgement...yet. But, to your question, if someone swears out affidavit for an order of protection, then yes, the cops are going to - and should - ‘hunt down an detain everyone.’ That’s how it works now.
If,OTOH, they simply picked up the phone and said: Hey, my kid is crazy, can you please do something?, that’s going nowhere. We’ll have to see exactly what was said to police and in what context.