As a police officer in a small Oregon town in 2004, Sean Sullivan was caught kissing a 10-year-old girl on the mouth. Mr. Sullivan's sentence barred him from taking another job as a police officer. But three months later, [he was hired] as the police chief ... in Cedar Vale, Kan., [where] he was again investigated for a suspected sexual relationship with a girl and eventually convicted on charges that included burglary and criminal conspiracy. Some experts say thousands of law enforcement officers may have drifted from police department to police department even after having been fired, forced to resign or convicted of a crime. Yet there is no comprehensive, national system for weeding out problem officers. A lack of coordination among law enforcement agencies, opposition from police executives and unions, and an absence of federal guidance have meant that in many cases police departments do not know the background of prospective officers if they fail to disclose a troubled work history. Among the officers ... who have found jobs even after exhibiting signs that they might be ill suited for police work is Timothy Loehmann, the Cleveland officer who fatally shot 12-year-old Tamir Rice in 2014. Before he was hired in Cleveland, Officer Loehmann had resigned from a suburban police force not long after a supervisor recommended that he be fired for, among other things, an inability to follow instructions. But Cleveland officials never checked his personnel file. Officer Loehmann, who was not indicted, remains on the Cleveland force.