(CNN)The calls for police officers to wear body cameras is not new.
They came after Michael Brown was killed in 2014; after Walter Scott was fatally shot in 2015 and they're ringing out again.
As the cases of Breonna Taylor, George Floyd and Rayshard Brooks reignite protests over police violence, federal legislators, state officials and even law enforcement are again pointing to body cameras as a part of the solution.
But the evidence on whether the technology actually reduces police use of force incidents or changes how communities view departments is mixed.
One study of the Metropolitan Police Department in Washington D.C. -- one of the largest, most rigorous reviews of its kind -- found that body cameras had no significant impact on officer use of force, on civilian complaints, on whether a case was prosecuted and other outcomes. Another study of the Las Vegas Metropolitan Police Department found that body cameras reduced use of force and civilian complaints only modestly.
"The key is to have realistic expectations about what body worn cameras can accomplish," says Michael White, co-author of "Cops, Cameras, and Crisis: The Potential and the Perils of Police Body-Worn Cameras." "... The one thing they can't do is fix decades of tension between police departments and the minority community."
The bottom line is this: Body cameras aren't an easy fix. Rather, experts say, they're only as effective as the departments that adopt them. And even then, they're just one piece of a very complicated system.
Getting cameras is easy. Maintaining them is expensive
By 2016, nearly half of law enforcement agencies in the US had acquired body-worn cameras, according to a Bureau of Justice Statistics survey. it's likely those numbers now are significantly higher because agencies have been quick to adopt the technology over the years, White said.
But acquiring body cameras is just the beginning.
"Buying the cameras and giving the cameras to officers is the easy part," says White, who is also a professor of criminology and criminal justice at Arizona State University. "Then it gets extraordinarily complicated and expensive."
- There's the cost of the cameras themselves -- which can range from about $400 to $800 apiece, depending on the camera make and model and how many a department orders.
- There's the high costs associated with storing the video footage recorded by the cameras, which can be about $15 to $99 per officer per month. States have varying laws on how long video that could serve as potential evidence must be preserved. For all other video footage, police departments generally set the policies, ranging anywhere from 30 days in Boston to five years in Atlanta, according to the Brennan Center for Justice.
- Then there's the personnel costs associated with maintaining the footage, says Steven Casstevens, president of the International Association of Chiefs of Police and police chief for Buffalo Grove, Illinois. Police departments need to have employees who are reviewing the videos, responding to public records requests and performing other administrative functions.
Police departments don't always have a clear plan for their use
For body cameras to be effective, a police department needs to have a proper plan in place.
The Department of Justice has a checklist that law enforcement agencies can consult as they develop policies around how body cameras are used. The International Association of Chiefs of Police also has a set of guidelines around what to consider -- such as when a camera should be activated, who reviews the footage, etc.
But in police departments around the US, that's not always the case.
"There are a variety of ways that a program can fail," White says. "Certainly one of the ways is that a department hasn't properly planned out the program."
At least 19 states and Washington, D.C. require police departments to develop written policies around body cameras, according to the National Conference of State Legislatures. In other localities, departments have taken it upon themselves to do so.
Departments acquire body cameras for a number of reasons. Some departments get them to improve officer accountability and community relations. Others may get them to enhance training, improve evidence quality or make cases easier to prosecute.