If Phil Cook backed his way into the study of gun violence in the U.S.—arriving at the topic via a broader study of criminal justice—Kristin A. Goss found herself thrown in headfirst. On the afternoon of April 20, 1999, Goss M.P.P. ’96, then a graduate student at Harvard, was standing in her Somerville apartment when she saw the face of a Denver anchorman on the television set. Goss had grown up in Colorado, and she recognized the anchorman.

On a Friday in December, Philip J. Cook received an email message alerting him to a mass shooting at a small school in Newtown, Connecticut. The details were startling: The perpetrator, a twenty-year-old later identified as Adam Lanza, had murdered his mother with one of her own handguns, before making his way to Sandy Hook Elementary School, where he killed twenty children and six staff members, including the principal and the school psychologist.