Franklin Zimring, who teaches law at UC Berkeley, had what sounds like a simple question: How many people do law enforcement officers kill each year in the U.S.?

The answer turned out to be anything but easy. Zimring explains the problems with the data and his findings in an upcoming article in the Ohio State Journal of Criminal Law.

Zimring, one of the country’s leading experts on criminal justice, searched in vain for reliable data. He ended up turning to Wikipedia as a source. He calls the lack of good statistics a scandal and is advocating for a national database of the killings.

Several recent high-profile deaths, including the fatal shootings of Michael Brown in Ferguson, Missouri, and 12-year-old Tamir Rice in a Cleveland park, and the death of Eric Garner in New York after a police chokehold, have prompted a national outcry and left many wondering how many similar deaths occur each year.

Political leaders across the spectrum are joining the clamor for better statistics on this issue. Recently, Attorney General Eric Holder called for better data on police shootings. He said gathering better data about shootings by police, as well as police officer deaths, is the “first step” toward ensuring officers’ safety and civilians’ rights. Republican Sen. Rand Paul supported the Death in Custody Reporting Act, a bill passed in December that requires states to report deaths of people who are killed during arrests or while in custody.

Zimring – whose article, “Trends in Killings of and by Police: A Preliminary Analysis,” will be published in the fall 2015 issue of the Ohio State Journal of Criminal Law – details the problems with the current data available on police killings.

He examined the supplementary homicide data in the FBI’s Uniform Crime Reports and “justifiable homicide” data – defined as “the killing of a felon by a law enforcement officer in the line of duty” – and found little information about the circumstances that led to such killings. Why did the police officer use deadly force? Did the victim suffer from a mental illness? If the police officer was reacting to the threat of an assault, did the other person have a weapon?

Two key problems emerged: The reports are voluntary, and there is no auditing mechanism.

In December, The Wall Street Journal tried to piece together a national database to compare with the FBI’s statistics. The newspaper found more than 550 homicides by law enforcement agencies between 2007 and 2012 that were not included in the FBI’s records. There have been efforts to crowdsource the data, and last week, KQED published an interactive map of California shootings by law enforcement that relied primarily on some of that data.

Among the states whose statistics are not currently included: Florida and New York.

At the Florida Department of Law Enforcement, spokeswoman Gretl Plessinger said the agency submits supplemental homicide information to the FBI but the state’s data does not fit the categories that the FBI uses. For example, Florida’s data lists “spouse” as an option under “victim relationship to offender,” but the FBI lists “husband, wife, ex-husband, ex-wife.”

A spokesman for the New York State Division of Criminal Justice Services said the agency plans to start reporting data this year. Walter McClure said the agency is “in the process of making technical fixes to its electronic crime reporting database.”

Police in Washington, D.C., did not report details on homicides in 1998 to 2008, the journal reported, and had spotty statistics in recent years.

The junior author on Zimring’s article, doctoral student Brittany Arsiniega, searched everywhere for narratives of these deaths. She ended up spending a lot of time on Wikipedia. One page, titled “List of killings by law enforcement officers in the United States, 2012,” gives brief narratives of deaths.

The researchers used these narratives to build their own statistics on police killings, with the caveat that these statistics are incomplete. They looked at cases where police officers had been threatened with assault, prompting them to kill.

In 73.3 percent of these cases, the person who was killed had a firearm, the researchers found.

Zimring is hoping there is enough political will to create a national system for reporting. He says he hopes that the system would require states to collect high-quality data, audited for accuracy, and that it would be able to force law enforcement agencies to comply.

As he put it, “What do they do if a police department says, ‘Screw you’?”

Samuel Walker, professor emeritus of criminal justice at the University of Nebraska, Omaha has spoken nationally on the need for police accountability, including testifying in January before the president’s Task Force on 21st Century Policing.

The attorney general should mandate a national database of killings by law enforcement, he said. If that doesn’t work, Congress should mandate a database, with federal funding to local law enforcement contingent on whether local agencies comply with turning over their data.

“It’s a way of giving the public good information on a very important issue,” Walker said. “We have data on traffic accidents. We have data on seat belts. We have data on smoking and on all sorts of cancer. Why not this?”

Abbie VanSickle is a reporter for Reveal, covering guns and legal issues. She started her journalism career at Florida's St. Petersburg Times (now the Tampa Bay Times), where she covered crime and breaking news for four years. VanSickle also has worked as a lawyer, practicing as a public defender in Seattle and as a human rights lawyer in China. She received her J.D. from UC Berkeley School of Law and her journalism degree from the Medill School of Journalism at Northwestern University. She was a Henry Luce scholar in Cambodia, where she worked on behalf of genocide survivors at the Khmer Rouge Tribunal. VanSickle is based in Reveal's Emeryville, California, office.