In recent years, killings of black men at the hands of police have led to intense scrutiny of the use of deadly force by officers. Citizens have marched in protest and departments have changed the way they train street cops. But even with that shift, prosecutors seeking to criminally charge officers still face many hurdles.

The standard of proof is higher than a normal self-defense case. Prosecutors typically consider themselves part of the law enforcement “team” with police, and they may even know the potential defendant. And when a case finally goes to trial, juries tend to be sympathetic to the daily challenges faced by officers on the street and are more inclined to vote “not guilty.”

The prosecutors of Hennepin County, Minn., are a good example. In the 17 years from 2000 through 2016, none of the 43 police shootings handled by Amy Klobuchar, who served as county attorney through 2006, and her successor, current County Attorney Mike Freeman, resulted in charges. In 2018, Freeman obtained an indictment for Mohamed Noor, a black officer who killed an unarmed white woman, and a jury last year convicted Noor of third-degree murder and manslaughter — the same charges filed Friday against Minneapolis Officer Derek Chauvin in the choking death of George Floyd.

“I cannot and will not,” Freeman said Friday after charging Chauvin, “allow a case to be charged until it’s ready. This case is now ready.” He said he was not ready to charge the other three officers at the scene of Floyd’s death Monday. At least two of them appeared in one video to also be restraining the handcuffed man suspected of passing a counterfeit bill. Floyd, who is black, was unarmed. Chauvin is white.

“There’s reluctance on the part of the citizenry,” said Michael Smith, a criminologist at the University of Texas-San Antonio, “as reflected in the decisions of prosecutors, to hold police officers accountable because of the unique nature of the role they have. There’s a leeway that we typically want to give police, doing a difficult job under difficult circumstances. So there’s a general reluctance for prosecutors to charge police for deaths in custody, and a reluctance for jurors to convict.”

Data gathered by The Washington Post shows that American police fatally shoot about 1,000 people a year, and an untold number of police killings occur by other means, such as the choking of Floyd. In many cases, officers act to save the lives of themselves or others. But even in instances where an officer’s actions are controversial, few cases end up judged in a courtroom.

In the 10-year period from 2005 to 2014, when perhaps 10,000 Americans were killed by police, 153 officers were charged, or about 1.5 percent, according to a database maintained by Philip Matthew Stinson at Bowling Green University.

There are known dispositions in 139 of those 153 cases, Stinson said. About 55 percent of the cases resulted in convictions, most by guilty plea. Stinson’s database also captures non-shooting cases. His research found 58 murder or manslaughter cases that did not involve a gun. Of the 51 cases with known results, about 65 percent resulted in a conviction, but only 47 percent of the cases ended with felony convictions.

“It’s rare that officers get prosecuted for on-duty deaths,” Stinson said, “especially ones that are not shootings.”

When a police killing occurs, authorities have two choices for possible prosecution: a murder or manslaughter case in state court, or a criminal civil rights violation in federal court. Minneapolis authorities have begun a state case against Chauvin, but federal prosecutors said they are also looking at a possible civil rights case.

Cynthia Deitle, the former chief of the FBI’s civil rights unit, said federal authorities will try to coordinate first with local law enforcement in the hopes that a state or county prosecutor can handle the case. “If we have faith that they are going to do the right thing, we will hang back, and offer them assistance,” said Deitle, now the director of civil rights reform at the Matthew Shepard Foundation. “Now that there are actually state charges filed against somebody, we’re not going first, we’re going to let the state handle it.”

If state prosecutors decline charges, as happened in the videotaped death of Eric Garner in New York City in 2014, federal prosecutors can then step in. In Garner’s case, the Justice Department investigated and then decided last year not to charge the officers involved. Cases of police misconduct handled by the Justice Department typically take far longer to resolve — an average of three years to file charges, in a review of more than 50 cases studied by The Post. And charges are declined in 96 percent of police misconduct cases considered by the Justice Department.

But more frequently, state prosecutors handle the case. Most states, including Minnesota, have laws that specifically define when a police officer may use lethal force. Minnesota’s law says officers may use deadly force only to protect themselves or another person from apparent death or great bodily harm, or to capture a person believed to have committed a felony involving deadly force.

Smith noted those are essentially the same legal arguments that any civilian can make when raising a claim of self-defense. But the U.S. Supreme Court has added two crucial rulings that frame the consideration of most police shootings.

In Graham v. Connor, the court ruled that an officer’s actions must be “objectively reasonable” under the circumstances at the time, as the officer perceived them then, without judging them in hindsight. And in Tennessee v. Garner, the court ruled that an officer may not use deadly force to prevent an escape unless the officer has probable cause to believe the suspect poses a significant threat of death or serious physical injury to the officer or others.

When prosecutors seek murder charges, against any defendant, they must also prove the defendant’s intent to kill, with different degrees of murder used for different circumstances. First-degree murder involves premeditation; second-degree murder is typically a more spontaneous killing. Minnesota is one of only three states with third-degree murder, which it defines as causing a person’s death “by perpetrating an act eminently dangerous to others and evincing a depraved mind, without regard for human life” but “without intent to effect the death of any person.”

Intent is also crucial, and influential for prosecutorial decisions, in federal civil rights cases. A federal criminal violation, called a “deprivation of rights under color of law,” makes it a crime to “willfully deprive or cause to be deprived from any person” their rights under the Constitution or federal law. The “objectively reasonable” standard is also used to analyze officers’ actions in federal court.

“What I had to prove in many of my cases was the officer knew what he was doing was wrong and he did it anyway,” said Deitle. “That’s not always easy, and we don’t know yet what the defense will argue in Minneapolis.”