ROSEBURG, Ore.— Since Christopher Harper-Mercer killed nine people at an Oregon community college on Thursday, Douglas County Sheriff John Hanlin has repeatedly stood before television cameras and briefed the public on the investigation into the horrific mass shooting. But he never once has uttered Mr. Harper-Mercer’s name.

Sheriff Hanlin is one of a growing number of U.S. law-enforcement officials who are actively avoiding naming the suspects in mass shootings, noting that many cite prior killers as inspiration and seem to...