Making a Murderer’s cops appear to have planted blood that, unlike this spatter, was very real (Photo: Kameleonklik/Deviant Art)

Making a Murderer, Netflix’s obsession-worthy true-crime show, has two fundamental tensions in it. Did Steven Avery kill Teresa Halbach? And did the police go to unjust and illegal lengths to make it seem like he had?

Whether Avery committed the crime or not, the show makes a strong case that the police did plant evidence, including blood in Halbach’s car, to make it easier to nail Avery with the murder. Even fans of the show who think Avery might be guilty believe that cops falsified evidence in the course of their investigation.

It’s one of the details that makes the documentary show so compelling: sure, cops plant evidence on overwrought TV shows, but in real life? It’s almost too dramatic to be real.

The truth is, though, that no one knows how often police officers plant evidence in order to make their jobs easier, amp up charges, or flat-out frame people for crimes they didn’t commit.

“It’s a hidden crime. There’s not much we know about it,” says Philip Stinson, a former police officer and professor at Bowling Green State University, who has built the best data set on police misconduct in America. “No government agencies track this kind of thing.”

When proof of planted evidence does come to light, it’s often because federal agents and prosecutors catch officers in city police departments. There are a just handful of cases like this in the United States each year; most that came up from a search in Stinson’s database involved planted drugs. And, occasionally, they were motivated by revenge.

Here are a few examples of how these cases generally go.

In New York City, in 2008, an officer was caught planting drugs on four men in a bar; he later testified that he was helping out a fellow officer who needed to make his arrest quota. “It’s almost like you have no emotion with it … they’re going to be out of jail tomorrow anyway; nothing is going to happen to them,” the indicted officer told a judge.

In Tulsa, Oklahoma, an FBI agent posed as Mexican meth dealer named “Joker,” who met with local cops in a Super 8 motel in 2009. The officers were eventually caught in a conspiracy to steal drugs and money from drug dealers. During the trial, other officers testified that the group had planted evidence up to 12 times, usually when the suspect had swallowed actual evidence. (Two of the officers were eventually acquitted.)

In Camden, New Jersey a group of officers were put on trial in 2013 for planting evidence; threatening people with arrest or charging people based on that evidence; taking drugs without reporting the seizure, in order to have drugs available to plant; and adding planted drugs to real evidence, in order to increase penalties in particular cases. In this case, all four officers were found guilty.

Sometimes police officers are motivated to plant drugs for more personal reasons. In 2011, for instance, a police sergeant in Texas planted meth in his ex-wife’s car: the couple was in custody fight for their children, and it seemed his intention was to ensure she lost. And in 2012, a judge in Georgia conspired with two officers to plant meth in the car of a woman who had previously filed a complaint against him, stating that he had “solicited her for sex in exchange for legal favors,” her lawyer said.

The case in Making a Murderer has a whiff of this same revenge motive: Avery had previously been imprisoned for almost two decades for rape, a crime for which he was exonerated. When he was arrested for Halbach’s murder, he was suing the police department for tens of millions of dollars.

These cases may be isolated examples of police misconduct. In one study of police ethics and corruption, the researchers call planting drugs a “serious form of noble-cause corruption”—an instance of police doing wrong for what they believe are the right reasons—but argue that “though some police may get involved in this, it is not very common.”

It’s hard to say anything for sure, though. When police officers are prosecuted for planting evidence, often they’re charged with “official misconduct” or violation of oath. “That hides the true nature of the crime quite often,” says Stinson.

And there are likely instances of planting evidence that are harder to see—the ones that are dealt with internally. “What I’m really interested in is when people don’t get arrested,” Stinson says. “We don’t know how often that happens.”