"Nineteen years ago, Dave Cullen was among the first to arrive at Columbine High, even before most of the SWAT teams went in. While writing his acclaimed account of the tragedy, he suffered two bouts of secondary PTSD. He covered all the later tragedies from a distance, working with a cadre of experts cultivated from academia and the FBI, but swore he would never return to the scene of a ghastly crime. But in March 2018, Cullen went to Marjory Stoneman...