It happened again: A white police officer accused of unnecessarily shooting and killing a black man was found not guilty by a court.

The latest case is in St. Louis, where protests erupted over the weekend as a result of the verdict.

There, a judge on Friday found former local police officer Jason Stockley not guilty of first-degree murder for the 2011 shooting of black motorist Anthony Lamar Smith. “This Court, in conscience, cannot say that the State has proven every element of murder beyond a reasonable doubt, or that the State has proven beyond a reasonable doubt that the defendant did not act in self-defense,” St. Louis Circuit Judge Timothy Wilson declared in his ruling.

Stockley shot and killed Smith after a police chase over an alleged drug deal, claiming that he feared the 24-year-old black man was reaching into his car to grab a gun. But prosecutors argued that Stockley had planted a revolver to justify the killing.

As the St. Louis Post-Dispatch reported, videos show Stockley going back to a police vehicle to go through a duffel bag. Then Smith, who’s dead at this point, is pulled out of his car, and Stockley goes into Smith’s vehicle. Prosecutors suggested that was when he allegedly put the revolver in Smith’s car.

Prosecutors claimed that Stockley can also be heard in police dashboard camera video saying that he’s “going to kill this motherfucker, don’t you know it.” Stockley’s defense insisted he never said this, and it is hard to make out in the video.

Tests found that the revolver only had Stockley’s DNA, not Smith’s. But several videos of the scene never captured Stockley actually carrying the gun.

Judge Wilson said he wasn’t convinced after “[a]gonizingly” going over the evidence. He argued that the gun was too large for Stockley to have successfully hidden it from the cameras, and, citing expert witnesses, that the lack of DNA evidence doesn’t mean Smith didn’t own the gun. And he said it would be strange if Smith didn’t have a gun, given that he was believed to be a drug dealer: “Finally, the Court observes, based on its nearly thirty years on the bench, that an urban heroin dealer not in possession of a firearm would be an anomaly.”

The verdict inspired protests in St. Louis, near where protesters also demonstrated against the police shooting of Michael Brown in Ferguson, Missouri, in 2014. Police arrested dozens at the demonstrations over the weekend, and at one point chanted “whose street, our street” — a mantra that’s commonly used by those protesting police violence — after they cleared a street on Sunday night.

For protesters, this is yet another example of a police officer getting away with needlessly shooting and killing a black man.

But the case also shows why it’s so difficult to punish cops for these shootings: According to the law, police officers only have to reasonably perceive a threat for a shooting to be justified — even if a threat isn’t actually there. Both Stockley and his partner claimed they saw a gun before they opened fire, which would be enough to legally justify a shooting under these standards.

Al Watkins, an attorney for Smith’s fiancée, Christina Wilson, urged everyone to “stay peaceful” after the verdict. But tensions remain high over racial disparities in American policing, with the court’s decision only adding fuel to the fire.

Protests broke out in St. Louis over the weekend

Protests were tense over the weekend, as demonstrators poured out into the streets of the St. Louis area to speak out against the acquittal of Stockley. Although most of the protests were peaceful, things occasionally got violent — with people breaking the windows of some businesses in the area and injuring several people.

According to the Guardian, hundreds went to the demonstrations over the weekend. They blocked off streets. They marched through different areas of the city, including two shopping malls in a wealthy area of the county after beginning in the local police’s headquarters.

But the protests followed a pattern similar to that of the Ferguson demonstrations. Most of the protests were peaceful throughout the day. By nightfall, most protesters had gone home. With about 100 or so protesters remaining, things then got rowdier and rowdier — even violent as some of the participants injured police officers at the demonstrations, knocked over concrete planters, broke windows, and tossed trash cans and other objects into the streets.

Police reinforcements came in through buses on Sunday. They scattered across the city in riot gear, making arrests. In total, more than 50 were arrested over the weekend.

Some of the protesters criticized those who got violent, arguing they don’t represent the full demonstrations. State Rep. Bruce Franks, who participated in the peaceful protests, told the Guardian that those who got violent “are not protesters” but separate from the more organized demonstrators. Protest organizer Anthony Bell said, “I do not say the demonstrators are wrong, but I believe peaceful demonstrations are the best.”

More protests are expected on Monday — all rooted in a belief that police are racially biased, particularly in how they use force.

Black people are much more likely to be killed by police than their white peers

Based on nationwide data collected by the Guardian, black Americans are more than twice as likely as their white counterparts to be killed by police when accounting for population. In 2016, police killed black Americans at a rate of 6.66 per 1 million people, compared to 2.9 per 1 million for white Americans.

There have also been several high-profile police killings since 2014 involving black suspects. In Baltimore, six police officers were indicted for the death of Freddie Gray while in police custody. In North Charleston, South Carolina, Michael Slager was charged with murder and fired from the police department after shooting Walter Scott, who was fleeing and unarmed at the time. In Ferguson, Darren Wilson killed unarmed 18-year-old Michael Brown. In New York City, NYPD officer Daniel Pantaleo killed Eric Garner by putting the unarmed 43-year-old black man in a chokehold.

One possible explanation for the racial disparities: Police tend to patrol high-crime neighborhoods, which are disproportionately black. That means they’re going to be generally more likely to initiate a policing action, from traffic stops to more serious arrests, against a black person who lives in these areas. And all of these policing actions carry a chance, however small, to escalate into a violent confrontation.

That’s not to say that higher crime rates in black communities explain the entire racial disparity in police shootings. A 2015 study by researcher Cody Ross found, “There is no relationship between county-level racial bias in police shootings and crime rates (even race-specific crime rates), meaning that the racial bias observed in police shootings in this data set is not explainable as a response to local-level crime rates.” That suggests something else — such as, potentially, racial bias — is going on.

One reason to believe racial bias is a factor: Studies show that officers are quicker to shoot black suspects in video game simulations. Josh Correll, a University of Colorado Boulder psychology professor who conducted the research, said it’s possible the bias could lead to even more skewed outcomes in the field. “In the very situation in which [officers] most need their training,” he said, “we have some reason to believe that their training will be most likely to fail them.”

Part of the solution to potential bias is better training that helps cops acknowledge and deal with their potential prejudices. But critics also argue that more accountability could help deter future brutality or excessive use of force, since it would make it clear that there are consequences to the misuse and abuse of police powers. Yet right now, lax legal standards make it difficult to legally punish individual police officers for use of force, even when it might be excessive.

Police only have to reasonably perceive a threat to justify shooting

Legally, what most matters in police shootings is whether police officers reasonably believed that their lives were in immediate danger, not whether the shooting victim actually posed a threat.

In the 1980s, a pair of Supreme Court decisions — Tennessee v. Garner and Graham v. Connor — set up a framework for determining when deadly force by cops is reasonable.

Constitutionally, “police officers are allowed to shoot under two circumstances,” David Klinger, a University of Missouri St. Louis professor who studies use of force, previously told Dara Lind for Vox. The first circumstance is “to protect their life or the life of another innocent party” — what departments call the “defense-of-life” standard. The second circumstance is to prevent a suspect from escaping, but only if the officer has probable cause to think the suspect poses a dangerous threat to others.

The logic behind the second circumstance, Klinger said, comes from a Supreme Court decision called Tennessee v. Garner. That case involved a pair of police officers who shot a 15-year-old boy as he fled from a burglary. (He’d stolen $10 and a purse from a house.) The court ruled that cops couldn’t shoot every felon who tried to escape. But, as Klinger said, “they basically say that the job of a cop is to protect people from violence, and if you’ve got a violent person who’s fleeing, you can shoot them to stop their flight.”

The key to both of the legal standards — defense of life and fleeing a violent felony — is that it doesn’t matter whether there is an actual threat when force is used. Instead, what matters is the officer’s “objectively reasonable” belief that there is a threat.

That standard comes from the other Supreme Court case that guides use-of-force decisions: Graham v. Connor. This was a civil lawsuit brought by a man who’d survived his encounter with police officers, but who’d been treated roughly, had his face shoved into the hood of a car, and broken his foot — all while he was suffering a diabetic attack.

The court didn’t rule on whether the officers’ treatment of him had been justified, but it did say that the officers couldn’t justify their conduct just based on whether their intentions were good. They had to demonstrate that their actions were “objectively reasonable,” given the circumstances and compared to what other police officers might do.

What’s “objectively reasonable” changes as the circumstances change. “One can’t just say, ‘Because I could use deadly force 10 seconds ago, that means I can use deadly force again now,’” Walter Katz, a California attorney who specializes in oversight of law enforcement agencies, previously said.

In general, officers are given lot of legal latitude to use force without fear of punishment. The intention behind these legal standards is to give police officers leeway to make split-second decisions to protect themselves and bystanders. And although critics argue that these legal standards give law enforcement a license to kill innocent or unarmed people, police officers say they are essential to their safety.

For some critics, the question isn’t what’s legally justified but rather what’s preventable. “We have to get beyond what is legal and start focusing on what is preventable. Most are preventable,” Ronald Davis, a former police chief who previously headed the Justice Department’s Office of Community Oriented Policing Services, told the Washington Post. Police “need to stop chasing down suspects, hopping fences, and landing on top of someone with a gun,” he added. “When they do that, they have no choice but to shoot."

Police are rarely prosecuted for shootings

Police are very rarely prosecuted for shootings — and not just because the law allows them wide latitude to use force on the job. Sometimes the investigations fall onto the same police department the officer is from, which creates major conflicts of interest. Other times the only available evidence comes from eyewitnesses, who may not be as trustworthy in the public eye as a police officer.

“There is a tendency to believe an officer over a civilian, in terms of credibility,” David Rudovsky, a civil rights lawyer who co-wrote Prosecuting Misconduct: Law and Litigation, previously told Amanda Taub for Vox. “And when an officer is on trial, reasonable doubt has a lot of bite. A prosecutor needs a very strong case before a jury will say that somebody who we generally trust to protect us has so seriously crossed the line as to be subject to a conviction.”

If police are charged, they’re very rarely convicted. The National Police Misconduct Reporting Project analyzed 3,238 criminal cases against police officers from April 2009 through December 2010. They found that only 33 percent were convicted, and only 36 percent of officers who were convicted ended up serving prison sentences. Both of those are about half the rate at which members of the public are convicted or incarcerated.

The statistics suggest that it would have been a truly rare situation if Stockley was convicted of a crime.