Licensed therapist and medical professionals are required by laws to report to the police if a patient under their care makes a threat of harm to people. It is their discretion what constitutes a credible threat but their license to practice will be on the line if they fail to report.
After reading the article above, I would probably not have reported Cruz to the authorities myself. It seemed they were constantly working to mainstream Cruz and the last entry by the therapist seemed somewhat promising, whatever that's worth.
Re: 17 killed in shooting at Parkland, Florida high school
276"It is better to be violent, if there is violence in our hearts, than to put on the cloak of non-violence to cover impotence. There is hope for a violent man to become non-violent. There is no such hope for the impotent." -Gandhi