A Wall Street Journal analysis of the latest data from 105 of the country’s largest police agencies found more than 550 police killings between 2007 and 2012 were missing from the national tally kept by the FBI or, in a few dozen cases, not attributed to the agency involved. The result: It is nearly impossible to determine how many people are killed by the police each year. Public demands for transparency on such killings have increased since the August shooting death of 18-year-old Michael Brown by Officer Darren Wilson in Ferguson, Missouri. The Ferguson Police Department has reported to the FBI one justifiable homicide by police between 1976 and 2012. Law-enforcement experts long have lamented the lack of information about killings by police.