Baltimore City state's attorney Marilyn Mosby announced on May 1st that six Baltimore police officers would be prosecuted for the death of Freddie Gray in police custody on April 19.

This isn't just a big deal for Baltimore, which has dealt with both peaceful and violent protests this week over residents' anger at the police treatment of Gray and the Baltimore Police Department's record of police brutality. It's a big deal nationally because it's incredibly rare for officers to ever go to trial for killing civilians — as seen in high-profile cases last year in Staten Island, New York and Ferguson, Missouri.

David Rudovsky, a civil rights lawyer who co-wrote the book Prosecuting Misconduct: Law and Litigation, is an expert on how and when police in America are prosecuted. After Ferguson, Missouri police officer Darren Wilson wasn't charged in the death of Michael Brown in November 2014, Rudovsky spoke to Vox's Amanda Taub about the "inherent conflict of interest" involved in holding police criminally responsible for potential misconduct on the job. Although the conversation focused primarily on Wilson's shooting of Brown, it's a good illustration of why what happened in Baltimore is the exception — not the rule.

Amanda Taub: There can be a sense that the investigations into police misconduct are different from civilian crimes — that the prosecution standards are different, as are the likely outcomes. Is that accurate?

David Rudovsky: That's certainly accurate. The number of criminal prosecutions of police, whether by the federal government or by state prosecutors, is very, very low. They are very rare. They are difficult, for reasons I'll get into. So I think that perception is right.

If I was involved in a situation like the one in Ferguson — if I was in a car and my gun was used and somebody was killed — very likely, I would be arrested very soon. I might have a good self-defense claim, but that would be a defense at trial. In a lot of situations where civilians might be fairly quickly charged with assault or harassment, police are not. Sometimes for good reasons. We obviously authorize police to use force, and deadly force. We actually let them use it where civilians cannot. There will be situations where they can use force and you and I could not.

the first inherent conflict is that the police are investigating their own

But there are obviously also institutional reasons why police are prosecuted so infrequently. So what you see is more care given to the investigation — or at least more time, because in a lot of situations it may just be a coverup to avoid charging the police. But even assuming good faith investigation, they'll take more time.

AT: Let's talk about some of these institutional reasons, starting with the police who investigate the incident.

DR: In many cases, the first inherent conflict is that the police are investigating their own.

Ferguson's a little different because you have a grand jury. But in many situations, it's often the police department that is charged with investigating a particular incident, deciding who's telling the truth, who used force first, and so on and so forth.

But even where it's done completely in good faith, or where an outside agency is brought in to investigate, such as a grand jury or the federal government, you're still talking about a pretty intense investigation.

I'm not suggesting that there's complete immunity for officers, just because they're officers

Inherent in those cases are all kinds of questions of credibility. Unless we've got a very clear video of what happened, it's often the word of the officer versus the person who may have been criminally assaulted. If the person was killed, we don't have his word at all, so what do the forensics show? What do witness statements show?

Race undoubtedly plays a factor, to the extent that it's a white officer and a black victim. In this country, people like to ignore that, but that undoubtedly plays a role.

There are some cases where it's absolutely so obvious that what the officer did could be a crime that we have a quick arrest, but not often.

AT: Can you give me an example of cases that have moved more quickly?

DR: Those are usually of the corruption type. Where it's absolutely clear — someone's caught on tape giving a bribe. Something like that.

I'm not suggesting that there's complete immunity for officers, just because they're officers. But the corruption tends to be dealt with differently than what I'd call the possible physical misconduct.

AT: Why is that?

DR: Departments take corruption more seriously than they do excessive force. It's often easier to prove. It's there, you see it, you have it. But oftentimes with use of force, you get a lot of different versions from witnesses.

AT: Do some police departments internally consider certain kinds of force acceptable, even if that's not consistent with what the public would think or the legal standards?

DR: There's a mix here. Part of what the police departments do, and how they operate, is a reflection of how a lot of people in society feel that police should be given a free hand.

There is a tendency to believe an officer over a civilian, in terms of credibility.

The more concern there is about crime, the more general political feeling there is that we have to give the police more powers and not be too rough on them if they cross the line slightly. I'd say the benefit of the doubt generally goes to the officer in close cases.

AT: Is it a popular sentiment that if police are not allowed to use force, that will put them in too much danger?

DR: It's central to this whole issue. Their position is that if you prosecute officers in a case like this, the next time the officer's going to hold back, and he's going to get killed.

It's this "split-second" argument: The officer's only got a split second to make a decision, so even if he makes a wrong one, why should we criminally punish him?

The other alternative message is that if you don't let them use that force, you're going to be less safe. More crime. More criminals. I don't necessarily agree with that, but that's certainly something you hear from the officers — and from other people as well.

AT: Let's move on to prosecutors. Are there conflicts or tensions when it comes to possibly prosecuting an officer from a department that they regularly call officers to testify from?

DR: Oh, sure. For example, that officer may have a dozen or more cases in which there are pending criminal charges — you charge that officer, those cases are going to certainly be called into question.

Beyond the individual officer impact, they work so closely on a day to day basis, it's very much like prosecuting one of your own. It's like prosecuting a fellow prosecutor. That doesn't mean that the right choice and decision isn't made in some cases. But it's an inherent conflict.

AT: Could prosecutors view police misconduct prosecutions as a barrier to obtaining a high-profile position, such as district attorney?

DR: In some localities, for sure. In certain areas, police and police unions are often very strong politically. And to get on the wrong side of the police is usually not a very good political move, so that may deter some prosecutors.

This is something that's particularly true in small communities. Even in larger cities, in places where they have very strong unions and strong support for police, it can be seen as a risky step — which is why the federal government should step in, in certain situations. They're independent.

But it's not easy for federal prosecutor to make that decision. They also rely on local police for some of their investigations, and the burden of proof in federal prosecutions of police is even higher than the state.

when an officer is on trial, reasonable doubt has a lot of bite

AT: Now let's turn to juries. You have mentioned that juries are often reluctant to convict officers. Why?

DR: There is a tendency to believe an officer over a civilian, in terms of credibility.

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.

AT: Are there particular defenses that you see that are effective with juries?

DR: You often see — and it's very effective — a case like Ferguson, where the officer's defense appears to be, "there was a scuffle and at one point he was going for my gun, and he was running, and I didn't know if he had a gun or not" or "I thought he had a gun; he had something metallic in his hand."

The argument is: "Okay, members of the jury, the prosecutor is saying 'he shot too quickly, he should have waited,' but the fact is that if he'd waited another three seconds he could be dead. The fact is that a lot of cops are killed in the line of duty, and sometimes they're killed because they wait too long."

That's an effective emotional argument.

AT: Are there things that you think could be done about this? Suggested solutions?

DR: There will always be these difficulties, but they can be mitigated. If prosecutors and the federal government took this more seriously and invested the time and resources that they do in other types of investigations, there would be more cases that would be properly charged and get convictions. There are convictions sometimes in these cases, so it's not impossible!

It's about our political will. It's a matter of consciousness and political will and taking police misconduct as seriously as we take other crimes.

AT: What's going to inspire that kind of political will?

DR: That's got to be forced from the bottom up. If people don't demand that, we won't get it.