More police officers have been charged over civilian killings in 2015 than at any time in the last decade

Twelve police officers have been charged with a crime this year stemming from fatal encounters with civilians while on duty.

That's a record that stretches back at least 10 years, according to data compiled by Bowling Green University criminal justice professor, Philip Stinson - an ex-cop whose data was cited by the Wall Street Journal.

The annual average number of prosecutions over that time period is five.

There's little doubt about the origin of this increase in prosecutions. Police killings in the last two to three years have been subjected to intensemedia and political scrutiny.

And with some groups pointing to a specific trend in such deaths - that they most often involve unarmed black men (and in some cases women or children) - it's not hard to understand why such an increase has occurred.

Protests following police-involved deaths in Ferguson, Missouri, Staten Island, New York and Baltimore, Maryland have cracked the veneer of inculpability sometimes afforded by the police badge, leading to more prosecutions of law enforcement officers.

But, more court trials do not necessarily lead to more convictions. Retired San Francisco Bay area district attorney Tom Orloff explained it this way in The Journal: "most jurors have the idea the cops are out to do the best job they can."

The prevailing assertion in these matters is, if a police officer uses deadly force against a civilian, it was done out of necessity.

Some have argued the opposite - asserting that police have been too quick to react with deadly force. The Washington Post notes that this year alone, several hundred people have been shot dead by police.

Despite the increased number of police officers tried in court for on-duty killings, "no single officer has been convicted of murder or manslaughter this year," the WSJ reports.

Several high-profile cases have been talked about of late, including that of former Charleston, South Carolina police officer, Michael Slager. Slager was denied bond earlier this month as he waits for his murder trial to begin. He's accused of fatally shooting an unarmed man named Walter Scott, who was running away from Slager during an encounter in April.

Six officers in Baltimore will face separate trials over the death of Freddie Gray, who suffered a medical emergency that severed his spine 80% at his neck while he was in police custody.

A handful of similar cases are pending.