WASHINGTON—When 24-year-old Albert Jermaine Payton wielded a knife in front of the police in this city’s southeast corner, officers opened fire and killed him.

Yet according to national statistics intended to track police killings, Mr. Payton’s death in August 2012 never happened. It is one of hundreds of homicides by law-enforcement agencies between 2007 and 2012 that aren’t included in records kept by the Federal Bureau of Investigation.

A Wall Street Journal analysis of the latest data from 105 of the country’s largest police agencies found more than 550 police killings during those years were missing from the national tally 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 police in Ferguson, Mo. 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. “When cops are killed, there is a very careful account and there’s a national database,” said Jeffrey Fagan, a law professor at Columbia University. “Why not the other side of the ledger?”