As protests continue across the U.S. over the Ferguson, Mo., grand jury finding, police oversight experts are reminding Americans that officers face different standards when it comes to criminal trials.

On Monday, a grand jury decided there wasn't enough evidence to indict a police officer in the shooting death of 18-year-old Michael Brown on Saturday afternoon, Aug. 9, in the St. Louis suburb of Ferguson, following a brief confrontation in the street.

After the grand jury announcement, protests and rioting erupted in the Missouri city, and large demonstrations spread quickly across the U.S. For many demonstrators, both the shooting and the failure to indict Officer Darren Wilson signified racial injustice and an irresponsible use of police force.

While policing experts acknowledge that there are legitimate concerns about the use of force by police, they stress that there are a number of factors that make charging an officer with homicide or excessive force in the course of duty difficult to bring about.

"I understand why people are concerned," said David Klinger, a Missouri criminologist. "My hope would be, however, that people look at the evidence rather than running to emotion."

Criminal proceedings involving police use of force must focus on whether the officer had reason to believe that either the officer or others were at risk of death or serious bodily harm. In these cases, an officer's belief is often what is key.

In the U.S., officers can also use deadly force if a dangerous suspect is fleeing.

Walter Katz, a California lawyer specializing in police oversight, notes that there's a "great deal of leeway" when it comes to the use of lethal force by police.

"A police officer can be mistaken with his perception, as long as it was reasonable," says Klinger, an associate professor at the University of Missouri-St. Louis.

Officers can only be accountable for what they thought they knew at the moment.

The public now knows that Brown was unarmed during the Aug. 9 shooting. However, during the grand jury hearing, Wilson seemed to suggest he thought Brown had a gun tucked in his pants. He testified that he fired at the 18-year-old when he saw Brown reach into his waistband.

That's apparently such a common occurrence by officers in lethal shootings that it's dubbed a "waistband shooting," Katz says.

Tricky to prove

In use-of-force hearings in the U.S., several factors are considered: the seriousness of the crime, the level of resistance a suspect puts up and the level of threat to the officer or public.

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Protesters continued to protest for a second night on Tuesday after an officer was cleared in the shooting death of Michael Brown. (Jim Young/Reuters)

Still, much of that is difficult to prove.

The police officer is "very often the only witness to the lethal use of force," notes Katz. And even if that is not the case, the officer's perspective carries a lot of weight.

In St. Louis, the 12-member grand jury not only heard extensively from Wilson, but also from about 60 witnesses on 25 days over the course of three months.

Most of these witnesses appeared to be in agreement with Wilson that the encounter began with a struggle at the window of the police vehicle.

Wilson said he fought with Brown over the officer's gun while he, Wilson, was still seated in the car, and when the gun first discharged. After that is when witness accounts diverged.

Some say the officer ran behind Brown and shot at him.

Wilson says he radioed for backup, then chased after Brown, saying it was to buy time until other officers arrived. That's when he says Brown turned around, reached toward his waistband and Wilson fired another volley of shots.

Push to reduce lethal force

Katz says he wishes the public wouldn't apply a broad brush to all police-related shootings — either painting them as murder or assuming they are all justified. "Every event is unique," he says.

Still, he acknowledges that too often officers resort to lethal force instead of using de-escalation tactics or non-lethal alternatives

When it comes to the criminal system, however, the onus is not to prove the officer had other alternatives to lethal force, only that lethal force was a reasonable alternative.

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For many demonstrators, both the shooting and the failure to indict Officer Darren Wilson signify racial injustice and unjustified police force in the United States. (Lucas Jackson/Reuters)

In the U.S., there are calls for better data collection so that issues such as unjustified police force and racial bias can be examined. It's currently impossible to know how often police resort to deadly force.

The FBI only keeps track of "justifiable homicides" by law enforcement officers, and says in its most recent report there were 410 such cases in 2012.

How Canada compares

Criminal trials for police shootings in Canada mostly focus on similar criteria as in the U.S., but that's where the similarity ends, says Rick Parent, a 30-year veteran of the Delta Police Department who has served as an expert in U.S. and Canadian courts.

"The two countries are vastly different when it comes to violence," said Parent, who is currently an assistant criminology professor at Simon Fraser University.

Police shootings and police murdered by assailants both happen more frequently in the U.S. on a per capita basis, says Parent.

Also, officers rarely face murder charges in a criminal court in Canada. More often, they face lesser charges of careless use of firearms, which signal lack of intent to kill.

But that may be changing.

In 2012, for the first time ever for Toronto, a police officer was charged with second-degree murder in the shooting death of a man during a drug raid. The charge was later thrown out.

More recently, a constable was charged with second-degree murder in the Toronto streetcar shooting of 18-year-old, Sammy Yatim.

Parent suggests that these recent murder charges against police officers may signal a change in how the Canadian legal system intends to handles them.