“West of Memphis” is now playing at Sundance Cinemas. R, 2:26, three stars out of four.

While watching “West of Memphis,” I kept thinking about another documentary about the criminal justice system I saw recently, one that probably won’t get as much attention as “Memphis” and its notorious case. “Gideon’s Army,” which played at the Wisconsin Film Festival last month and will be on HBO in July, looks at three overburdened public defenders in Southern courts. It depicted a criminal justice engine that was designed to elicit guilty pleas out of defendants, particularly poor defendants, whether they are actually guilty or not.

Because that’s pretty much how the historic “West Memphis 3” case ended, a shocking miscarriage of justice in Arkansas in which three teenagers were railroaded into being convicted of the murders of three 8-year-old boys in 1992. The case has now been the subject of four documentaries, and the teens (now in their late 30s, having spent 18 years in prison) had celebrities like Eddie Vedder, Natalie Maines and “Lord of the Rings” director Peter Jackson fighting on their behalf. But at heart, right to its troubling end, it’s another case where, instead of a prosecution having to prove a case, a defendant has to choose the lesser of two evils. “This happens all the time,” defendant Damien Echols says of the whole process, and he’s right.

The three “Paradise Lost” documentaries aired on HBO covered the twists and turns of the case as they happened, but now Amy Berg’s fascinating and maddening “West of Memphis” gets to look back on the entire arc of the case. Berg is no disinterested observer in the case (Echols serves as producer, as did Jackson, who bankrolled an investigation to clear the three defendants’ names). But largely, Berg lets the evidence tell the story, meticulously examining what might have happened on that night when three boys went missing, only to turn up dead in a river the next morning, bound and bludgeoned. (Berg includes some horrifying crime scene photos as she sifts through the evidence, so this film is not for the faint of heart.)

From the start, the investigation by local police reeked of incompetence; mistaking post-modem wounds by swamp turtles as some sort of ritual mutilation, the police jumped on the idea that this was some sort of Satanic murder, and focused on Nichols and the other two because of what they wore, what they listened to  and what they wrote in their notebooks. It’s shocking how seemingly important players in the case were simply never interviewed by police, so sure they were of their theory. But with some misleading “experts” and a jailhouse informant willing to testify on behalf of the state for a lesser sentence, convictions were secured.

But advocates continue to fight the case, and DNA evidence points squarely at the stepfather of one of the boys, who had a history of abuse. If this were a TV show, the real killer would confess in dramatic fashion. But this is real life, and he walks free. Instead, largely to save face after such an embarrassing miscarriage of justice, a complicated plea agreement is worked out that allows the three defendants to claim innocence while still entering a guilty plea.

Again, just get a guilty plea on the books. This happens all the time.