One day after 19-year-old Nikolas Cruz used an AR-15 to kill 17 people in Parkland, Florida, last Wednesday, Broward County Sheriff Scott Israel ended a news conference by proposing an idea that is sure to trouble free-speech and civil-liberty activists: He called for lawmakers in D.C. and Tallahassee to pass laws letting cops detain civilians just for posting “questionable” things on social media, including photographs of knives or vague comments about admiring serial killers, even if the people writing the posts aren’t actually threatening anyone.