National statistics show that hundreds of homicides committed by law-enforcement officers between 2007 and 2012 were not recorded in the FBI's Uniform Crime Report, the Wall Street Journal reports.

More than 550 homicides committed by police during that period were missing, the paper reports. The lack of complete data makes it impossible to accurately determine how many people police kill each year.

Demands for more transparency on such killings have been shoved into the spotlight after the August shooting death of 18-year-old Michael Brown by officer Darren Wilson in Ferguson. The Ferguson police department has only recorded one justifiable homicide between 1976 and 2012, according to statistics.

Local police departments are not required to participate in the FBI’s uniform crime reporting program.

Some agencies tend to not report the killings, Bureau of Justice statistician Alexia Cooper told the journal. Nearly 800 agencies reported about 2,400 killings by police, while more than 18,000 other departments did not report any.

Some entities in the reports said they did not view justifiable homicides by law-enforcement officers as something that should be reported. Some agencies did not consider the events to be actual offenses.

In certain cases, if an officer killed someone in a city or town out of its jurisdiction believed that particular town would handle the report, by they had not done so.

In recent years, police have tried to rely on the data to develop better tactics in policing.

A particular alarming report came as recently in Washington D.C.

Police in Washington did not report any details about any homicides to the FBI for an entire decade starting in 1998; the same year the Washington Post revealed the city had one of the highest officer-involved killings in the country.

The city reported five killings by police in 2011, but zero in the following year after 24-year-old Albert Payton was killed by police while wielding a knife.

Significant increases in officer-involved killings can spark questions about management within the police department, Mike, a criminologist at Arizona State told the journal. “Sometimes that can be tied to poor leadership and problems with accountability.”