The dean of the Columbia University Graduate School of Journalism said that a four-month investigation of a now-discredited Rolling Stone article about an alleged gang rape at the University of Virginia found no evidence that magazine staffers committed clearly fireable offenses. Rolling Stone retracted the November article “A Rape on Campus” by Sabrina Rubin Erdely late Sunday after the publication of a 13,000-word Columbia review requested by the magazine. At an hour-long news conference on Monday, the school’s dean, Steve Coll, explained the report’s findings. When asked by reporters why no one was fired in the wake of the investigation,...