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I understood the article I read to have said she actually had it in her car in the parking lot.

I don't understand why the guards hadn't done anything as soon as she was past them, why the building wasn't on lock-down, why no one was announcing an active shooter in the building over the intercom... it took one brave employee to run around behind her and holler for people to flee at the great risk to his own life and safety.

She sounds like she had paranoid delusions. While other things can also have the same symptoms, the mention of her flat affect and her troubled history would point to schizophrenia being a likely diagnosis. Schizophrenia is usually a late-onset disorder, and on average has an older onset in women than in men (women in their late twenties to early thirties for onset, men throughout their twenties). Most schizophrenics are not violent, but her history of interpersonal conflict would put her in the risk category for being among the small population who are, especially if she had trouble in school, low IQ, or early drug use as well.

It does seem like there were a lot of ignored red flags.