When can police officers use force, and when is use of force excessive?

Those questions hang in the air after the family of Jonathon Ferrell, an unarmed 24-year-old man shot ten times by Charlotte police officer Randall Kerrick last September — and pronounced dead at the scene — filed a wrongful death lawsuit in civil court on Monday.

There are no hard national standards, no binding state policies, not even a national database that tracks how often, where, and under what circumstances police use deadly force. The result, say scholars, is a free-wheeling space in American law and police policy. The nation’s 17,000 law enforcement agencies set their own terms—and when citizens cry foul, the courts spit out wildly inconsistent results.

"Pick up the paper any day and there’s an excessive force case here and an excessive force case there, and yet there’s no national data at all," says William Terrill, a professor of criminal justice at Michigan State University. That contributes to a larger problem of excess subjectivity, he says, where cops who commit brutality can end up going free — guilty of what Terrill calls "lawfully awful behavior."

The legal test for excessive use of force, according to a 1989 Supreme Court ruling, is whether the police officer "reasonably" believed that the force he or she used was "necessary" to accomplish a legitimate goal. But there are no universal definitions of "reasonably" or "necessary," and in a drive from one jurisdiction to another, the court standards — and the local police department policies — can shift as briskly as songs on the radio.

"Excess is in the eyes of the beholder," adds Terrill, a former military police officer. "To one officer ‘objectively reasonable’ means that if you don’t give me your license, I get to use soft hands, and in another town the same resistance means I can pull you through the car window, I can tase you."

That leaves the lower courts to spin their own "ad-hoc, often inconsistent, and sometimes ill-considered" conclusions, says Rachel Harmon, a professor of law at the University of Virginia. They support a bullet fired in Newark, perhaps, and find a similar shot unconstitutional in Trenton. The result, she says, is a national patchwork, one where "many unconstitutional uses of force go uncompensated and undeterred."

This soft spot in the law and the data exists at a time when police shootings seem to be on the rise in many cities.

Charlotte police killed five people last year, the most in a decade—but less, City Manager Ron Carlee told the Charlotte Observer, than were killed by police in Washington (49), Memphis (42), Fort Worth (32), or Austin (17), all of which have seen their own numbers creep upward.

Boston police (along with their counterparts at the state level) have fired more bullets in each of the last five years, hitting at least 23 people last year, 11 of them fatally. Philadelphia police shot 52 people in 2012, prompting the commissioner to ask the U.S. Justice Department for a special review. Dallas, Miami, Baltimore, Chicago, Houston, Las Vegas, Los Angeles, New York City: all have been rocked by use of force scandals in recent years.

But does any of this add up to a national problem?

No one knows for sure.

The FBI is a clearinghouse for national crime data, from break-ins on the San Francisco Bay to stolen property on the streets of New York. But while it issues an annual report on law enforcement killed or assaulted by civilians, it doesn’t do the same for civilians unjustifiably killed or assaulted by police.

In 1994, Congress required the Attorney General to "acquire data about the use of excessive force by law enforcement officers," and "publish an annual summary" of these data."

"It was never implemented," says Terrill. The Justice Department did not return a request for comment. The FBI, meanwhile, acknowledged the shortcomings in its data.

"The data collected is general in nature," said FBI spokesperson Stephen G. Fischer Jr., in an email to NBC News, "and does not allow for the level of detail to determine the occupation of the offender."

To help fill the data gap, Terrill and colleagues recently completed a national survey of police use of force policies, examining how those policies translate to outcomes on the street. He got more than 600 departments to participate, and he isolated eight more—including Charlotte — for a deeper look, including a survey of officers and a two year review of crime, complaints and instances of police violence.

He went looking for what amounts to a standard policy, and hopefully an ideal one, but what he found was jarring: there’s no such thing as a standard policy — and nothing currently in use is ideal. More than 80 percent of departments train officers on a use of force "continuum," running from less lethal to more lethal, depending on the level of civilian resistance. Within that continuum, however, there is variety allowing for "nearly all types of force against nearly all types of resistance."

"Departments pick and choose, tweak and adapt [their policies], in a multitude of ways," Terrill and his colleagues wrote. "All unfortunately with no empirical evidence as to which approach is best or even better than another."

Based on the survey results, Terrill picked eight comparable cities for a closer look, and he won cooperation from Charlotte, Portland, Albuquerque, Colorado Springs, St. Petersburg, Fort Wayne, and Knoxville. That offered him a good mix of policies on police violence. And by stacking one policy against another, he hoped to isolate the best one, something researchers have never been able to do before.

The result could bolster the case argued by the family of Ferrell, the former football player killed by Charlotte police last summer. In fact, when contacted by NBC News, Professor Terrill acknowledged that he had just been retained by the family, who hoped to incorporate his results — and possibly his testimony — into their case.

Terrill concluded that Charlotte police officers had one of the worst policies under review. The city’s officers didn’t use force more often, but when they did use it they injured suspects at by far a greater rate, 73 percent compared to 45 percent for the next city on the list. Charlotte’s officers were also among the most likely to point their guns, and compared to the other cities, they used more force "relative to citizen resistance." Charlotte’s officers also rated their city’s force policy as comparatively unhelpful and vague.

Charlotte police did not respond to a request for comment on these findings. But Chief Rodney Monroe has defended his city’s police training as "adequate," even as his department has called Kerrick’s actions "excessive," and supported the state’s charge of voluntary manslaughter.

In a twist that’s typical in use of force cases, Terrill says, his findings may actually help Kerrick beat the criminal charges against him. The fact that Charlotte’s officers may be more likely to draw their guns, injure their suspects, and feel unassisted by their own policies than similar cities may help suggest that what Kerrick did wasn’t the result of bad faith — but bad policing, which isn’t in itself a crime.

"It’s an odd world we live in," says Terrill. "I don’t understand it myself."

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