A grand jury on November 24 decided not to press charges against Ferguson, Missouri, police officer Darren Wilson for the shooting of Michael Brown. The grand jury has regularly met since August 20 to investigate the death of Brown, a black 18-year-old who was unarmed when Wilson fatally shot him on August 9. Ferguson residents and supporters of Brown have protested regularly since the shooting, which they view as another example of police brutality against black youth. An indictment for Wilson has been one of their primary demands.

How did we get here?

Wilson stopped Brown on August 9 for jaywalking, according to Ferguson Police Chief Tom Jackson. A few minutes later, Wilson shot Brown to death.

The details of exactly what happened between the initial confrontation and shooting are unclear. The publicly available evidence from police and eyewitness accounts suggests there was a brief physical confrontation at Wilson's SUV. Brown then ran, and soon after that, Wilson killed Brown. Wilson's reported account claims that Brown reached for his gun and made him fear for his life during the initial confrontation. The eyewitnesses interviewed by the press seem to agree that Brown was shot while he had his hands up. (St. Louis County Attorney Robert McCulloch later claimed that many of the public eyewitnesses proved to be unreliable, because they changed their statements.)

The shooting almost immediately triggered protests in Ferguson, where people filled the streets and took up the chant "hands up, don't shoot." The protesters were largely peaceful, especially after some rioting on the first Sunday of demonstrations led to a concerted effort by leaders to keep people calm. But police at first reacted with military-grade equipment, in some cases using weapons like rubber bullets and tear gas on peaceful demonstrators and journalists, and arresting protesters and members of the media alike.

The protests largely dwindled in early September. They picked up again toward the end of the month, when a Brown memorial mysteriously burned down and Ferguson's police chief issued an apology that many locals saw as too little, too late. Protests were further renewed by Ferguson October, a movement activists launched on October 10 to highlight "the epidemic of police violence facing Black and Brown communities."

The protests have continued to fluctuate since then. For the most part, they've appeared to centralize on one goal: Wilson needed to be indicted and taken to court.

How do grand juries work?

During a grand jury, people are presented with evidence to decide whether someone — in this case, Wilson — should be charged with a crime. If they choose to indict, the case goes to court. If they don't, the case effectively dies.

Grand jury proceedings are typically secret. Participants are supposed to stay quiet about the hearings, even after a decision is reached. Leaks during the hearings are grounds for restarting the entire process.

But St. Louis County Attorney McCulloch later announced that he will release all the evidence presented to the grand jury if it decided not to indict Wilson.

The prosecutor has almost complete control as to what evidence the grand jury sees and hears. In the case of Wilson, McCulloch promised that "absolutely everything will be presented to the grand jury. Every scrap of paper that we have. Every photograph that was taken."

Since prosecutors have so much control over the evidence, getting an indictment is generally perceived as easy. As Vox's Amanda Taub wrote, "The joke within the legal profession is that a decent prosecutor could get a grand jury to indict 'a ham sandwich.'"

But some argue McCulloch, who protesters see as biased due to his family ties to law enforcement, never meant to get an indictment — and actually launched the grand jury proceedings as a delay tactic.

"Prosecutors have the responsibility to make tough decisions about cases. Here, the district attorney should review the evidence, consult with investigators, and decide whether he believes there is probable cause to charge Wilson with a crime," Alex Little, a former federal prosecutor, told Taub. "The idea that the district attorney has to wait for the grand jury to spend weeks combing through the evidence is flat wrong, and if his office is promoting that explanation for its delay in acting, then it's being deliberately misleading."

When is a cop allowed to shoot a civilian?

Two Supreme Court decisions in the 1980s, Tennessee vs. Garner and Graham v. Connor, set the legal 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, told Vox's Dara Lind. 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 explained, comes from Tennessee vs. 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."

For the grand jury in Ferguson, the question was whether Wilson was aware that Brown allegedly took part in a robbery prior to the shooting. Wilson said he became aware during the encounter that Brown was a robbery suspect and radioed the realization in as soon as he knew.

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.

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 survived his encounter with police officers, but who was treated roughly, had his face shoved into the hood of a car, and broke 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 said 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. "The moment that you no longer present a threat, I need to stop shooting," said Klinger. According to the St. Louis County Police Department's account, Wilson fired one shot from inside the police car. But Brown was killed 150 feet away, after several shots had been fired. To justify the shooting, Wilson would need to demonstrate that he feared for his life not just when Brown was by the car, but even after he started shooting. The officer would need to establish that, right up until the last shot was fired, he felt Brown continued to pose a threat to him whether he actually was or not.

"One can't just say, 'Because I could use deadly force 10 seconds ago, that means I can use deadly force again now,'" said Walter Katz, a California attorney who specializes in oversight of law enforcement agencies.

Will this escalate the Ferguson protests?

Since the Ferguson protests were so focused on getting an indictment, it's almost certain the grand jury's decision not to indict will aggravate a lot of demonstrators. And government officials and businesses appear to be preparing for the worst.

Even before the grand jury decision was announced, Missouri Gov. Jay Nixon declared a state of emergency to mobilize the state's National Guard in Ferguson — a move that triggered criticisms from protesters and, according to the Washington Post, US Attorney General Eric Holder.

In a letter reported by St. Louis news station Fox 2, the owner of a shopping complex wrote, "We are asking every store manager to give some serious thought to what your response would be in the event of demonstrations or civil unrest." Other stores have reportedly boarded their windows, according to ABC News.

The Jennings School District, which includes the eastern edge of Ferguson, canceled classes on Monday and Tuesday of the week of Thanksgiving in expectation of a grand jury ruling. Ferguson-Florrisant School District later canceled classes on Tuesday.

Law enforcement took some precautions as well. The FBI sent teams of agents and officers to the area prior to the protests, according to NBC News. And local police geared up for potential unrest, the Huffington Post's Ryan Reilly reported: "The St. Louis County Police Department has stocked up on tear gas, less-lethal ammunition and plastic handcuffs in anticipation of massive protests in the suburb of Ferguson, Missouri, if a grand jury doesn't indict the police officer who killed 18-year-old Michael Brown."

What happens to Darren Wilson now?

There are several ways Wilson could still end up facing trial. It's possible that the state attorney general could bring criminal charges against Wilson, regardless of the grand jury decision. A federal investigation, led by the Justice Department, is also looking at whether Wilson violated Brown's civil rights by shooting and killing the unarmed teen, but recent leaks reported by the New York Times and Washington Post suggest the feds don't have the evidence to prove Wilson willfully violated Brown's civil rights.

Barring any formal charges, the Brown family could file a civil lawsuit against Wilson or the Ferguson Police Department. And Ferguson Police Chief Jackson vowed to conduct an internal investigation should Wilson not face charges.

It's possible Wilson won't return to the Ferguson Police Department regardless of the grand jury decision. People reportedly close to Wilson told the New York Times that Wilson has no intention of returning to the police department and is negotiating the conditions of his leave with city officials.