Dallas city officials have added extra encryption and other security measures to the outdoor warning sirens hacked early Saturday. The city believes the hack came from the Dallas area, but officials haven’t detailed how it occurred. The hack caused all 156 emergency sirens to activate for about 90 minutes, scaring some residents and doubling the number of calls to 911. Experts theorized the incident may have been a simple “replay attack” where the hacker recorded the radio signal sent out on April 5 at noon as part of a monthly test.”]