Unfortunately, parents, teachers, co-workers, friends, etc can see someone degenerating into mental illness and still not be able to do anything. Rules in this country for forced treatment start with a 72 hold for psychiatric evaluation. But this hold can only be forced if someone is an immediate danger to themselves or others. Getting a court order of commitment without some violence is almost impossible.
Almost every city and states lacks inpatient facilities. Insurance companies don't want to pay for inpatient treatment and all the state hospitals have been closed. Many of the mentally ill end up housed in jail.
Even if you do get the diagnosis and help, continuing treatment is almost always a problem. Many of the mentally ill do not recognize the fact that they are ill, and will stop taking meds when they feel better.
So a college professor can say "man that guy Fred behavior is getting really odd, I think he needs help", and all it will do is get Fred kicked out of school, probably making the condition worse. Or the neighbors can call the police and say Fred is acting erratically and is armed. When they police show up, yep Fred's acting weird and he does have guns; but the guns are legal and Fred isn't threatening anyone, so the police leave.
Every time someone says "they should have done something, they knew he was acting strange or was mentally ill", I do this:
Because the families involved probably had tried to get help, the teachers had probably said something to someone, and the co-workers had tried to help; but their hands were tied, and in the end the only thing they could do was mourn and wish the system worked better.