Eric Garner and Mike Brown are dead, and the men who killed them walked free. Why?

The failure of grand juries in New York and Missouri to indict the police officers who killed Garner and Brown—both unarmed black men—is a heartwrenching demonstration of the extent to which the American legal system treats police officers differently than civilians. Why is that? What does it mean? And what options do we have if grand juries and prosecutors fail us? Below, our guide to the complicated world of prosecuting cops.

What Is Excessive Force?

In theory, excessive force is defined as "any force beyond what's necessary to arrest a suspect and keep police and bystanders safe." But that conclusion is often complicated, because it all depends on the officer's state of mind.

So, in practice, shooting an unarmed suspect or putting a suspect in a chokehold isn't excessive force—legally speaking—when an officer believes the situation is life-threatening. It's not an objective standard, so the jury doesn't have to believe that the cop was actually in danger—just that he reasonably believed he was.

And this is exactly also where the institutionalized racism starts to creep in—cops seem to disproportionately fear for their lives around black people.

Why Don't Cops Get Indicted?

For the most part, cops don't get indicted because the criminal justice system treats cops differently.

When a possible felony has been committed, prosecutors usually bring charges by presenting evidence to a grand jury. The jurors determine two things: 1) whether there is probable cause to believe a crime was committed, and 2) whether the defendant committed that crime. It is, as they say, fairly easy to obtain an indictment. Except when the defendant is a cop.

One reason is because, by law, officers are allowed to use "whatever force they reasonably believe is necessary to make an arrest or to protect themselves." This standard means it's irrelevant if the suspect is unarmed, or deaf, or just playing with a toy at the park—as long as the police officer perceived there was a danger, it's generally not a crime.

For example, a Kentucky grand jury recently declined to indict a Sheriff's Deputy who killed a a 19-year-old preschool teacher last spring because they found the officer, Tyler Brockman, "feared for his life." The teacher, who had a .12 BAC, was driving away from a party when she accidentally ran over Brockman's foot. The officer then jumped on the hood of her slow-moving car and shot her through the windshield four times at point blank range.

In these types of shooting cases, officers "often testify that they perceived a deadly threat and acted in self-defense. This stance can inoculate them even if the threat later turns out to be false," criminologist Geoffrey P. Alpert told the New York Times.

What Role Do Prosecutors Play?

Another reason officers are so rarely indicted is the close relationship between the District Attorney's office and the police department.

"The district attorney's office works way too closely with the local police department and individual officers to be able to objectively look at these cases," a lawyer for the family of Jonathan Ferrell—a 24-year-old former football player who was killed by police as he tried to get help for his broken down car—told the New York Times.

When it comes to grand jury hearings, prosecutors act as an orchestra conductor, emphasizing the evidence they like and highlighting certain witnesses by calling them to testify. That's why it's so easy to indict defendants—the prosecutor can shape the story however he or she wants, and it's the only side the grand jury gets to hear. Via the Times:

For most felonies, grand jury hearings are swift, bare-bones proceedings. Prosecutors present enough evidence to show it is probable that the defendant, who rarely testifies, committed a crime, and ask the jury to vote for an indictment. Several cases are usually processed in a single day.

But not when the defendant is a cop.

But because most prosecutors impanel a special grand jury to investigate police-related deaths, they insulate themselves from the final decision, while appearing to fulfill the public desire for an independent review, legal experts said. The inquiries often go on for weeks or months, with testimony from several witnesses.

During the proceedings, offending officers are routinely allowed to tell the jury their version of the story, express remorse, and emphasize their adherence to training—all without having to be cross-examined. This is a courtesy rarely extended to civilian defendants.

It's not hard to see that reluctance to prosecute in recent high-profile cases. When the Ferguson grand jury decision came down, St. Louis County prosecutor Bob McCulloch presented an hour-long case for Wilson's innocence in a speech broadcast across the country.

In the Eric Garner grand jury proceedings, which concluded last weekend, the prosecutor limited the jury to two potential charges: criminally negligent homicide and manslaughter. In almost every felony case, a prosecutor will also ask the jury to consider what are known as "lesser included charges"—less serious charges that arise out of the same set of facts or circumstances. But Staten Island District Attorney Daniel Donovan chose to leave those charges—in this case, reckless endangerment—off the table. He has not explained why.

And that's when the case even goes before a grand jury.

In August 2010, a police officer there pulled up to a homeless man carrying a knife and ordered him three times to drop it. When the man didn't immediately comply, the officer shot and killed him. The man with the knife was partially deaf, and the knife was a woodcarving tool. Seattle's firearms review board ruled the shooting unjustified. The officer resigned but did not stand trial because the district attorney ruled there was not enough evidence to convince a jury he had acted with malice.

Las Vegas police aren't under a consent decree, but they were headed there after an investigation by the Las Vegas Review-Journal newspaper in 2011 exposed a culture in which unnecessary shootings were always found to be justified.

James Young, a 49-year-old father of three, fell into a coma and later died after he was choked by a narcotics detective "for an extended period of time" until he started foaming at the mouth and lost consciousness, a lawsuit claims. The Brooklyn district attorney declined to prosecute the case, and the civil lawsuit was settled for $832,500 in June.

A mentally ill woman, Shereese Francis, 29, of Queens, died of heart failure in 2012 after being suffocated by four male officers who were called to get her in an ambulance because she hadn't been taking her meds, another lawsuit claims.

After a 14-month investigation that included several home visits and interviews with family members who were present on the day of the death, the Queens DA declined to prosecute, according to court records. The family was awarded $1.1 million in a civil lawsuit in May.

A prosecutor cleared two Utah police officers of criminal charges in the killing of a young black man wielding a samurai sword, saying Monday the shooting was justified because the man had swung the sword at the officers.

Two Saratoga Springs police officers fired a total of seven shots, Utah County Attorney Jeff R. Buhman told reporters, hitting Darrien Hunt, 22, six times — at least once in the back as he ran away. Buhman's office investigated the Sept. 10 incident for nearly two months.

What Does It Take to File a Civil Suit Against a Cop?

Civil actions are a little friendlier to plaintiffs. As in a criminal case, the question again is whether the force was reasonable. But the plaintiff must only prove misconduct by a preponderance of the evidence, an easier evidentiary standard to attain.

Even so, the deck is often stacked against the victim suing a police officer. First, a plaintiff must show that the officer isn't shielded by the doctrine of qualified immunity, which prevents a plaintiff from suing a cop for actions that "fall short of violating a clearly established statutory or constitutional right."

It's not always an easy standard to meet.

For example, this summer, the Supreme Court held that an Arkansas cop who fired 12 shots into a car that crashed during a police chase cop was immune from a wrongful death lawsuit because "the use of deadly force to end a dangerous high-speed chase was constitutional and did not violate any statute."

And even when there is a basis for a suit, plaintiffs must jump through additional hoops to sue an officer:

Many states treat excessive force cases somewhat differently than typical lawsuits. In some jurisdictions, there is a presumption that the officer acted with the necessary level of force that the plaintiff must overcome. Additionally, some impose a higher burden of proof than "preponderance of the evidence," instead requiring the plaintiff to prove a claim of excessive force by "clear and convincing evidence" (a standard higher than "by a preponderance of evidence" but lower than "beyond a reasonable doubt").

When can the Justice Department step in?

When police misconduct is systemic and involving multiple victims, the Justice Department can step in by launching a "pattern or practice" probe. Authorized by a 1994 law, the DOJ has the power to sue police departments and cities that it concludes have systematically deprived citizens of their constitutional rights.

They generally focus on at least one of two things—police misconduct and civil rights violations. In misconduct inquiries, conduct like "excessive force, discriminatory harassment, false arrests, coercive sexual conduct, and unlawful stops, searches or arrests," are investigated. In civil rights investigations, investigators look one step further, for discrimination based on "race, color, national origin, sex, and religion."

The Justice Department is currently investigating police shootings in Ferguson and St. Louis, New York, and Fairfax.

But the investigating body—the Department of Justice Special Litigation unit—is small, and moves slowly. And when the Justice Department does find violations, it's only the first step.

In Cleveland, for example, it took almost two years to generate a report on the city police department's "pattern of excessive force." And it could be years before any changes are made to the police department—first Justice and the city will have to negotiate a consent decree, a process that could stretch into the 2020's, Cleveland.com reports.

The DOJ recently negotiated similar consent decrees—which allow the city to settle the charges without admitting guilt—in nine cities, including Albuqurque, New Orleans, Newark, New Jersey, Portland, Oregon, and Seattle. But as they enact new policies with criminal consequences, not all cops are receptive to the changes. In Oakland, for example, a federal judge had to place the entire police department under a receivership "after nine years of non-compliance." Cops in Cincinnati took seven years to adopt their new regulations, and hundreds of Seattle police officers sued the DOJ last spring, claiming rules requiring they "use only the force necessary to perform their duties," were slowing them down. The officers lost the suit, which all things considered, is a start.

[Illustration by Jim Cooke, handcuff photo via Shutterstock]