Darren Wilson killed Michael Brown. Here’s why he probably won’t go to jail.

Will Ferguson, Missouri, police officer Darren Wilson ever be prosecuted for shooting Michael Brown?

Since August 9, when Wilson shot and killed Brown, an unarmed teenager, protesters have taken to the streets of the St. Louis suburb and cities around the United States to insist that Wilson be charged in Brown's death. Their demand, emblazoned on T-shirts, inked onto handmade signs, and voiced in chants of "no justice, no peace," speaks to a widely held fear that the courts and police will be more interested in protecting the rights of a white police officer with no disciplinary record than the rights of a black 18-year-old.

Local and federal investigations into the shooting are underway, and a St. Louis County grand jury has already begun to hear evidence about the case.  That does not mean that a conviction, or even a trial, is guaranteed. The justice system is still a system: there are rules that must be followed, steps that must be taken, and criteria that must be fulfilled. Here's what you need to know.

Wilson could be prosecuted in federal court, state court, or possibly both

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Wilson could theoretically be prosecuted in state court, in federal court, or both, because state and federal authorities have concurrent jurisdiction over the shooting. At the moment, both state and federal investigations appear to be moving forward.

A state prosecution would, in some ways, be simpler. If the evidence supports a criminal case, state authorities could prosecute Wilson for murder or manslaughter, or just assault, which may be easier to prove than a federal civil rights offense, said Kevin Curran,  president of the Missouri Association of Criminal Defense Lawyers. "The state just has to show that he pulled the trigger, and he intended to pull the trigger, and there weren't any defenses." A federal prosecution, by contrast, would have to prove an additional element: that Wilson willfully deprived Brown of his civil rights.

Still, some aspects of a state prosecution might be difficult. State and local prosecutors work closely with the police, on whom they regularly rely for testimony in their cases. Aggressive prosecution of police misconduct can risk jeopardizing that relationship, which may make prosecutors reluctant to move forward cases involving police misconduct. In addition, the state prosecutors may face evidentiary problems if the St. Louis County investigation was insufficient. (That problem could possibly be solved by access to the results of the FBI investigation, however.)

Simple murder is not a federal offense, but it is a federal crime for a police officer to deprive someone of his rights under the Constitution. If a victim dies, the perpetrator can be sentenced to life in prison or even the death penalty. That means the DOJ could prosecute Wilson under federal law for violating Brown's civil rights, if the evidence supports that charge.

Because of a doctrine called "separate sovereignty," successive state and criminal prosecutions do not violate the double-jeopardy clause of the Constitution, which usually prohibits trying someone more than once for the same crime. That is why, for instance, the Los Angeles Police Department officers who assaulted Rodney King could be tried and convicted in federal court after they were initially acquitted in the Los Angeles Superior Court.

Federal civil rights prosecutions are rare, though, and convictions are even rarer. A study from Syracuse University's TRAC program found that between 1986 and 2003, fewer than 2 percent of civil rights matters referred to the DOJ were ever prosecuted. Out of 43,331 referrals, 690 were actually prosecuted — and of those, 423 resulted in a conviction.

The St. Louis County prosecutor has already sent the case to a grand jury, but his strategy is controversial

Police guard the Clayton, Missouri, courthouse where a grand jury has begun hearing evidence about the shooting of Michael Brown. (Scott Olson / Getty Images)

The St. Louis County attorney's office began presenting evidence about the shooting to a grand jury on August 20. That means that the state case probably won't move forward unless the grand jury votes to indict, which won't happen for a while — if it happens at all.

St. Louis County Attorney Robert McCulloch has said that "absolutely everything will be presented to the grand jury. Every scrap of paper that we have. Every photograph that was taken." As a result, he expects the grand jury investigation to last at least until October.

Grand jury investigations are secret. Not only are they closed to the public, grand jurors are not permitted to reveal the evidence that they heard. In a grand jury proceeding, the prosecutor presents evidence about the case, including witness testimony, and asks the grand jury to determine whether an indictment is warranted.

The prosecutor has almost complete discretion as to what evidence the grand jury hears. There is no obligation to present defenses or alternative theories of the case, and because the grand jury is not an adversarial proceeding, there is no cross-examination of witnesses. In Missouri, a grand jury vote in favor of an indictment does not have to be unanimous. If nine out of twelve grand jurors vote to indict, then that is considered a "true bill."

As a result, it is generally considered to be easy for a prosecutor to get a grand jury to indict. The joke within the legal profession is that a decent prosecutor could get a grand jury to indict "a ham sandwich."

That doesn't mean that will be the case this time. McCulloch's involvement in the case has already been highly controversial, with many observers doubting his dedication to prosecuting it. More than 100,000 people have signed an online petition demanding McCulloch's removal. Petitioners say his decision not to bring charges in a previous shooting, in which police officers killed two unarmed black men, as evidence that his continued involvement in the Brown case "will only sow further distrust and discord." On August 15, St. Louis County Executive Charlie Dooley announced that he would lead an effort to remove McCulloch from the case. But a few days later, Missouri Gov. Jay Nixon (D) announced that he would not ask McCulloch to step down from the case, citing a need to limit "legal uncertainty."

McCulloch's grand jury strategy has also provoked criticism.  Alex Little, a former federal prosecutor with experience working on FBI investigations, said that the prosecutor's decision to present all of the evidence, and to take such a long period of time, suggests that he could be using the grand jury as a "delaying tactic."

Courts tend to be sympathetic to police, which makes police brutality convictions difficult to obtain

Demonstrators in St. Louis rally in support of Darren Wilson. (Scott Olson / Getty Images)

If the grand jury returns an indictment, a conviction may be unlikely — juries are notoriously reluctant to convict police officers in use-of-force cases.

The precise reasons for that phenomenon are difficult to know because jury deliberations take place in private, which makes them hard to study. (A group of University of Chicago researchers secretly recorded several civil jury deliberations in the 1950s, but their work provoked such outrage that Congress passed a law making such eavesdropping illegal.)

But many experts have noted that police brutality cases turn the usual logic of a criminal trial on its head, by making the police officer the alleged "criminal" and the "criminal" the alleged victim (regardless of whether the victim is an actual criminal). Judge Guido Calabresi, in a discussion of civil suits in police brutality cases, wrote that "jurors are considerably more reluctant to identify with a criminal defendant who brings a tort action against the police for violation of his rights," because "in these cases, the plaintiff is a criminal and the jurors do not see themselves in that way." Jury sympathies are most likely even stronger in criminal cases, where the standard of proof is "beyond a reasonable doubt," not just the preponderance of the evidence that's generally required in civil cases.

Curran said that Missouri courts tend to be sympathetic to police, even in cases involving officers' questionable use of force. There is a sense among prosecutors, police, and juries alike, Curran said, that "cops have to be free to do their job, and it's a dangerous job, and they're under threat, so they have to have the freedom to be able to respond to perceived threats." Juries tend to look favorably on officers' claims that their use of force was necessary, because they "lean towards the officer's right to be safe." As a result, Curran said, local courts are "all basically pro-police."

Whether Wilson gets prosecuted or convicted will probably depend on whether his claims to self-defense are credible

Demonstrators in Ferguson raise their arms and shout, "Hands up, don't shoot." (Joe Raedle/Getty Images)

Police officers are subject to different rules about the use of deadly force than ordinary citizens. As my colleague Dara Lind explained at length, a police officer is allowed to use deadly force in two circumstances, both of which require the officer to determine that the target poses a threat to others. The first is when the officer believes that the target is directly threatening him or another person. The second is when the officer has probable cause to believe that the person is a suspect fleeing the scene of a violent felony.

That rule comes from a Supreme Court case called Tennessee v. Garner, in which the majority opinion explained:

"The use of deadly force to prevent the escape of all felony suspects, whatever the circumstances, is constitutionally unreasonable. It is not better that all felony suspects die than that they escape. Where the suspect poses no immediate threat to the officer and no threat to others, the harm resulting from failing to apprehend him does not justify the use of deadly force to do so. It is no doubt unfortunate when a suspect who is in sight escapes, but the fact that the police arrive a little late or are a little slower afoot does not always justify killing the suspect. A police officer may not seize an unarmed, nondangerous suspect by shooting him dead."

That means that for Wilson's shooting to have been legally justified, he will need to credibly argue that he believed either (1) Brown was threatening his life or someone else's life, or (2) that Brown was fleeing from the commission of a violent felony.

The latter argument seems less likely to be successful than the first one. Although Brown apparently was a suspect in a robbery of a local convenience store, it is not clear whether Wilson was even aware of that at the time he shot Brown. And, although the Ferguson police chief has described the alleged crime as a "strong-arm robbery," it involved no weapons, and the only physical contact was when the man who appears to be Brown shoved a clerk on his way out of the store.

For Wilson to take advantage of the "violent felon" exception, he would need to show that he knew about the robbery, that he believed Brown had committed it, and that he believed it had been a violent crime that made Brown a threat to the community. That means that Wilson would probably need to present evidence of police radio calls that described Brown as the suspect and the crime as a violent one.

Initial police statements suggest that Wilson is more likely to pursue a self-defense strategy. According to the Washington Post, Wilson told investigators that he shot Brown out of fear for his life when the teen "charged at him." St. Louis County Police Chief Jon Belmar said during a news briefing that Brown and Wilson had a physical altercation, during which Brown tried to grab Wilson's gun before running away.

Every bit of evidence matters

An FBI agent walks by a makeshift memorial at the site of the Michael Brown shooting. (Joshua Lott / AFP via Getty Images)

In the days since the shooting, tremendous federal law enforcement resources have been devoted to investigating Brown's death. The FBI sent a team of more than 40 agents to Ferguson, and they have been canvassing the neighborhood where the shooting took place. Little, the former federal prosecutor, told me that this is "a massive number of agents for a case like this." In a standard investigation into a police officer's use of force, Little said, you would have just two or three agents, which means that "40 is just exponentially larger than you would expect."

That's good, Little noted, because witness testimony and physical evidence are particularly important for prosecutions of this nature. Brown cannot testify about what happened, because he is no longer alive. So the testimony of eyewitnesses, and physical evidence from the scene, will be vital to determining what actually happened.

In this case, initial police statements suggest that Darren Wilson's defense will be that he shot Michael Brown in self-defense, after the teenager attempted to grab Wilson's gun during a physical struggle. Since Wilson's gun was fired at least once from inside his car and Brown was more than 30 feet away from the officer when he died, physical evidence will be able to provide some insight into how the shooting occurred and from what range various shots were fired. Already, preliminary autopsy results show that Brown was shot at least six times, and that he was facing Wilson when the shots were fired.

But only eyewitness testimony will be able to answer other questions, such as whether anyone saw Brown grab Wilson's gun, and whether Brown surrendered before Wilson fired the shots that killed him. That means that it's vital for those witness statements to be collected as part of the investigation.

Journalists may have filled a gap in the investigation

Demonstrators in Ferguson demand justice for Michael Brown. (Joshua Lott / AFP via Getty Images)

Although the FBI is now gathering evidence, the case is also being investigated by the St. Louis County Police, who took it on at the request of Ferguson Police Chief Tom Jackson. (Although the county prosecutor has already begun to present evidence to the grand jury, that does not necessarily mean that the county investigation is complete.)

It appears that the county investigators were slow to begin gathering evidence. They did not interview a key eyewitness in the case, Brown's friend Dorian Johnson, until Wednesday, August 13, according to Johnson's attorney. But it's really best to interview witnesses immediately, because eyewitness testimony tends to decline in quality over time. "What you want," said Little, is for "people to give statements that are memorialized very soon after the shooting."

The best way to do that is for witnesses to be interviewed by police. But some of the witnesses gave detailed interviews to the press shortly after the shooting. These interviews, Little said, could be the next best thing to actual police statements, because they recorded the witnesses' testimony soon after the shooting. If the witness sticks to what they said on TV in the trial, it will "help their credibility," he said.

Media interviews can also be risky. If witnesses change their stories over time, then the statements they gave to the press can be used to "impeach" their testimony — to call attention to the inconsistencies between the witnesses' testimony and the way they told their stories previously. If that happens, it will undermine their credibility.

Other members of the Ferguson Police Department could be prosecuted for conspiracy to deprive Brown of his civil rights

A protester in Clayton, Missouri, holds a sign supporting Darren Wilson and the police outside the courthouse in which grand jury proceedings are taking place. (Joe Raedle / Getty Images)

Although the federal investigation will likely focus primarily on Wilson's conduct as an individual, there is also a federal conspiracy statute that would allow the DOJ to prosecute other Ferguson police officers — or the Ferguson Police Department as a whole — if the investigation determines that they conspired to deprive Brown of his civil rights.

In order to support a conspiracy prosecution, the federal investigation would need to uncover evidence of intentional wrongdoing, not just incompetence. For instance, if there is evidence that the other officers who responded to the scene after the shooting prevented Brown from being resuscitated so that he would not be able to testify, or destroyed evidence in order to protect Wilson, then they could be prosecuted on conspiracy charges.

However, at this stage, a conspiracy prosecution appears to be unlikely. Although there was apparently no attempt to resuscitate Brown, the New York Times reports that paramedics did respond to the scene and examine him, where they found that he had suffered "injuries incompatible with life." And although some of the department's actions following the shooting seem questionable — such as leaving Brown's body in the street, at times uncovered, for four hours — that could just be the result of mistakes or disorganization, rather than intentional wrongdoing.

Read more on the shooting of Michael Brown

The New York Times called Michael Brown "no angel." Here's how it described serial killers

Hands up, don't shoot: The images that define Ferguson's protests

Ok, fine. Let's talk about 'black-on-black' violence

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