Allegations of the use of excessive force by police officers have engendered widespread outrage across communities and calls for deeper accountability and transparency in police work.

But U.S. law enforcement officers are rarely charged with homicide as a result of their actions or conduct in the line of duty, even though thousands of people are killed each year by police officers. Even fewer are ever convicted.

Consider these three recent examples:

A Burlington, Iowa police officer earlier this month was cleared of any wrongdoing in the fatal Jan. 6 shooting of a 34-year-old woman. Investigators found that

A December fatal shooting of a motorist by police during an overnight traffic stop was a tragedy but not a crime, Philadelphia District Attorney Seth Williams said last week. A grainy video surveillance captured some of the Dec. 15 incident in which

Police experts say public opinion has simply leaned in favor of police.

"In large part it's because the system recognizes that police officers are making split-second decisions in circumstances that are often very heated," said criminology professor Sean P. Griffin, interim department head of criminal justice at The Citadel in Charleston, S.C.

"We get the benefit of taking our time and walking through from 300 feet doing microscopic surgery on their detailed decisions. There's always deference to police. If you are assigning blame or motive, civilians are more willing to give police the benefit of the doubt."

The issue is front and center in central Pennsylvania this week after the Dauphin County District Attorney charged Hummelstown police officer Lisa Mearkle, 36, with criminal homicide in the Feb. 2 death of David Kassick, of South Hanover Township.

Police say Kassick, 59, fled from Mearkle when she tried to stop his vehicle for an expired inspection. She shot him with a stun gun, and as he lay prone on the ground, shot him twice in the back. Her actions were captured by a video camera mounted on her Taser.

Courts and juries have long given law enforcement agents wide discretion in the execution of their duties - and the benefit of the doubt. The proliferation of video footage showing encounters in which police take lethal action against a person may have begun to shift public attitudes away from police, but even in cases where video evidence is available, civilians tend to side with the police officer.

That's one of the main reasons it is difficult to prosecute or convict a police officer of homicide.

"If you don't have evidence to the contrary - to what the officer is saying, or officers are saying how a drug deal went down and the only people saying the opposite are the offenders or the offenders' friends or people who were there, the court systems historically defer to the police officers," said Griffin, who has authored books on organized crime and police matters.

Video evidence may cast doubt on a police officer's testimony, but courts or juries rarely rule against a police officer.

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"They give them the benefit of the doubt," Griffin said. "They are in circumstances we can't imagine. We aren't pulling someone over in the middle of the night and having that person reaching for something in the dark. It makes sense that we give them the benefit of the doubt in many of these cases."

In the Mearkle case, Dauphin County District Attorney Ed Marsico says it is the first time in his 15 years in office that he has leveled criminal homicide charges against a police officer.

"Today speaks volumes ... where the evidence shows that charges should be filed, we will not hesitate," he said on Monday at a press conference. "When police break the law, we will charge them accordingly."

Mearkle, who is free on a $250,000 bail and has been suspended from the Hummelstown force. The criminal homicide charges give prosecutors a variety of options in pursuing charges ranging from involuntary manslaughter to first-degree murder.

Mearkle arguably has her job description in her favor.

Police officers are authorized to use force and, when necessary, deadly force, says Mary Catherine Roper, a staff attorney at the American Civil Liberties Union of Pennsylvania's Philadelphia Office. Assessments of the use of deadly force by police typically center around whether the action violates the police department's policies - and only rarely leads to a criminal charge.

"To say a police officer murdered someone - that is intended to murder someone - that's a very different thing to say," she said. "To say 'Officer you misjudged the situation, you shouldn't have used more force,' that is not the same thing as saying you deliberately killed somebody. That's a very big accusation to make."

In the majority of cases, investigators find that the officer was justified in using the lethal action, most often in order to protect themselves or innocent bystanders from death or serious bodily harm.

"Police use high level of force - like shooting someone - in a tiny fraction of the percentage of all arrests," said Emanuel Kapelsohn, a firearms training expert and use-of-force consultant. "It's a rare occurrence. Most of the time the circumstances and evidence of whatever sort is available make it clear that the officer was justified in using force."

Determining just how often these incidents happen is hampered by the absence of comprehensive records as to how many lethal force police incidents there are each year. The 18,000 law enforcement agencies nationwide for the most part willingly self report deadly use of force to federal authorities; they are not mandated by law to do so. Experts widely agree that those figures woefully under represent the actual number of people killed by police.

The closest assessment available of the number of police officers charged with homicide comes by way of a report by Bowling Green State University criminal justice researcher Philip Stinson, who found that between 2005 and 2011, 41 U.S. police officers were charged with either murder or manslaughter in connection with on-duty shootings. Over that same period, the Federal Bureau of Investigation reported 2,718 justified homicides by police.

Another snapshot is provided by the U.S. Department of Justice, which found that fewer than 8 percent of complaints of excessive force are upheld against police by their departments. In its most recent analysis, the department found that in 2002 approximately 26,000 complaints of excessive force were lodged by citizens but in only 2,000 did police departments agree the officer used excessive force.

It warrants note that according to the FBI, 76 law enforcement officers were killed in line-of-duty incidents in 2013. Of these, 27 officers died as a result of felonious acts; 49 died in ac cidents. In addition, 49,851 officers were victims of line-of-duty assaults. The bureau's 2013 Law Enforcement Officers Killed and Assaulted report provides a comprehensive analysis of these incidents.

The fact is the courts have given police wide latitude in making the split-second decision about whether to pull the trigger and kill the suspect to save themselves or others or be killed.

Kapelsohn, who routinely testifies in court on behalf of police, says a police officer looking down the muzzle of a gun pointed by a suspect may have less than half a second to react if that gun is fired.

"The Supreme Court said decades ago that officers are often required to make split-second decisions under circumstances that are tense, uncertain and rapidly evolving," Kapelsohn said. "The Supreme Court said we have to judge this based on what a reasonable officer at the scene would do. We don't judge it as a Monday morning quarterback at the office."

The single most important factor determining whether a police officer walks away from a dangerous confrontation is his or her willingness to pull the trigger without hesitation, Kapelsohn said.

MORE: Was Lisa Mearkle justified in using deadly force?

"Most often I've seen officers die in confrontations because they hesitated to shoot when clearly it was necessary for them to shoot," he said. "As a police instructor, one of the hardest things for me to try to teach and train is the balance between the willingness to fire during a brief window of opportunity when if the officer doesn't fire, he or her partner or bystander or victim may be killed - and knowing that if she fires, this maelstrom is going to befall them."

Yet although encounters that result in police shooting a suspect happen most often because the officer says he or she feared for their lives and that the suspect was reaching for something, ostensibly a weapon, Roper makes the case that in most cases, the police officer was wrong.

The Department of Justice concluded just that this week in a year-long assessment of the Philadelphia Police Department. Among a myriad of shortcomings within the department, federal agents faulted Philadelphia police for having "failure in threat perception."

"I don't know what was going through this officer's head when she shot the man in Hummelstown, but in reviewing the footage, it sure looks like her claim that she thought she was in danger was not reasonable," Roper said. "She made error in threat perception. That is a much bigger problem. It isn't about bad cops, or racist cops, it's about police officers who are not given the training to react, to de-escalate, to find an alternative other than shooting someone because someone is not following your instructions."

In the case of Mearkle, Marsico said the decision to level the charges were largely based on video footage, which show Kassick prone on the ground, his hands only briefly hidden from her view. Police say Mearkle feared that he was reaching for a gun, when after repeated orders from her for him not to move, he made some type of motion with his arms.

Marsico acknowledged the role this case plays in the larger public discussion of police and civilian violence.

"You could also make the argument that the climate has never been more challenging for police officers," Griffin said. "Not to charge a police officer in this sort of circumstance, there's never been more scrutiny on police than right now."

What Marsico has not articulated, Griffin said, is whether he decided to charge Mearkle because he was cognizant of the growing scrutiny on police departments or whether he wanted to avoid a Ferguson situation under his watch.

"You just don't know," Griffin said.

That Marsico charged Mearkle is startling on its own, said D. Brian Burghart, editor and publisher of Reno News & Review, an online magazine devoted to covering deadly police force. District attorneys seldom take that step, he said, mostly because they are the wrong person to be tasked with the decision.

"He's the top law enforcement officer," Burghart said. "He's their representative but he is also doing the investigation. He's the guy who sends the guy with the arrest warrant. He's a friend. When is he ever able to step outside that role as friend and co-worker to indict someone? Not very often."

Marsico did not respond to requests for an interview for this story.

While both Mearkle and Kassick are white, the issue of lethal police force has most recently been centered around racial profiling. In the wake of last year's shooting of Michael Brown in Ferguson, Mo., and the choke-hold death of Eric Gardener in Staten Island, reams of studies have concluded that young black males are 21 times more likely to be shot by police than their white counterparts. An October 14 report by ProPublica found that the 1,217 deadly police shootings from 2010 to 2012 captured in federal data show that blacks, age 15 to 19, were killed at a rate of 31.17 per million, while just 1.47 per million white males in that age range died at the hands of police.

Kapelsohn says that while the race card does not apply in Mearkle's case, he is concerned that police officers are going to be criminally charged with greater frequency to "make up" for Ferguson and Staten Island.

"It's like when a referee makes a bad call and tries to make up for it," he said. "That's a concern. I'm not saying the police officer shouldn't be charged in this case, but on a nationwide basis, there's great concern among police that they are now going to be held accountable for things that are unfairly attributed to them."

The majority of police officers, Kapelsohn argues, avoid shooting a suspect at all cost, knowing full well that doing so will unleash a tide of departmental proceedings, civil and criminal court actions, compromise pension and career, and generate threatening emails and phones from people camped out on their front lawns.

"I personally worked as an expert in a case where a criminal with a large knife ran straight to the police officer," he said. "The officer instead of firing, ducked his head between his arms. Thankfully, the guy ran past him. I asked 'Why didn't you fire?' and he said, 'You know, I was going through a divorce right then and I said the last thing you need is more legal problems.' "

Griffin is certain that whatever jury takes the Mearkle case, its members will take into account what police face each day on the job and show deference to her.

"I have no reason for the jury not to be fair to her," he said. "Juries are notoriously fair to police unless there really is a huge change going around the country where people are upset by the images on TV or print news and juries at this time are less inclined to be favorable to police."

Editor's note: This story was updated to remove an example that was based on a hoax created by a satirical website.