Hundreds of homicides committed by police in the United States are not included in national crime statistics gathered by the FBI, an analysis of data shows.
The analysis of internal data from more than 100 of the country's largest police departments between 2007 and 2012 showed more than 500 police killings that were absent from the FBI's Uniform Crime Report, the Wall Street Journal is reporting.
Local police departments are not required to participate in the FBI program, and in addition killings involving federal officers are not in the FBI data.
That makes it almost impossible to accurately determine the number of people killed by police in any given year, and that's an unfortunate situation, law enforcement experts say.
"When cops are killed, there is a very careful account and there's a national database," says Columbia University law Professor Jeffrey Fagan. "Why not the other side of the ledger?"
Some local law enforcement agencies do not consider justifiable homicides by law-enforcement officers as something requiring reporting to the FBI, experts say.
"Does the FBI know every agency in the U.S. that could report but has chosen not to? The answer is no," says Alexia Cooper, a statistician with the Bureau of Justice Statistics who studies the FBI's data. "What we know is that some places have chosen not to report these, for whatever reason."
While nearly 800 agencies reported about 2,400 killings by police, more than 18,000 other departments did not report any.
While some local police enforcement agencies do report raw statistics, many don't include detailed information on individual homicides, which is the only way the FBI can track particular types of killings, including those involving police officers.
Demands for increased transparency on killings committed by police in the line of duty have grown following the shooting of Michael Brown in Ferguson, Mo., in August by officer Darren Wilson.
Many police officials have also called for better reporting, saying more accurate statistics would help police departments develop better tactics for dealing with possibly deadly situations.
"You want to get the data right," says Mike McCabe of the Oakland County Sheriff's Office in Michigan, explaining it is "really important in terms of how you deploy your resources."
Some police departments may be hesitant to report killings by their officers because such data might indicate problems with the department, says criminologist Mike White of Arizona State University.
"Sometimes that can be tied to poor leadership and problems with accountability," he says.