they didn't view the disturbing videos. ten minutes after they first arrived, deputies left. >> law enforcement's primarily a reactive component. not always. you know, we like to see them become more proactive. >> reporter: he assesses threats at work places. mental health workers would join officers in cases like this and would have immediate access to the web. >> they'll postinformation about a possible attack or a possible behavior prior to it occurring and this is -- if you're aware of this information, i think it would be ideal to look at it. >> reporter: in this case, it was a missed chance. but there were others. the gunman's former roommate tells abc news, saw warning signs. but didn't react.