It was after a late screening and the group of several hundred was aggrieved, its moral hackles raised.

"How could they treat him so badly?" one audience member asked.

"An injustice," said another.

"What can people do with their frustrations?" asked a third.

The setting was neither a courtroom nor an activist meeting. It was at the Sundance Film Festival, and the assembled had just watched "The Internet's Own Boy," Brian Knappenberger's quietly evocative look at wunderkind hacker Aaron Swartz, whom the film suggests was driven to suicide in 2013 by a zealous federal prosecutor in Massachusetts.

"We have real problems with our justice system," Knappenberger told the audience. "We've given a lot of power to prosecutors. Our system needs an overhaul."

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Ever since Errol Morris' landmark 1988 "The Thin Blue Line" led to the freeing of death row inmate Randall Adams, modern nonfiction films have been making the case for the wrongly accused. But in the age of rampant surveillance and unauthorized wiretapping, the U.S. documentary appears to be entering a new era, one in which the prosecutorial overreach film is not just the blood-boiling exception but a prevalent theme.

At the recently concluded Sundance — with four of the five 2014 Oscar nominees originating there, the festival is a key barometer of the documentary zeitgeist — nearly a half-dozen films contained this undercurrent. They included a wide range of subjects, in movies revisiting the case of Pamela Smart (the New Hampshire woman serving a life sentence for conspiring to kill her husband in 1990), the recently convicted Boston gangster Whitey Bulger and even an instance of the FBI confiscating a discovery by South Dakota paleontologists as prosecutors racked up charges against them.

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"You walk away from these films thinking that in our system of justice there are three parts," said Thom Powers, who programs documentaries for five film festivals. "There's a trial, then an appeals process, then there's a documentary."

By injecting themselves into the legal process, these films — many will play on TV and in theaters in the coming months — have opened a powerful storytelling vein and shone a light on perceived injustices. At this Edward Snowden moment, creators say, they tap into perceptions of government run amok. But they also create tricky conditions for filmmakers and the form.

Story twists

When Joe Berlinger decided last year to make a film about Whitey Bulger he hardly wanted to exonerate the mobster. In pursuing the case, though, he slowly began to see a more complicated portrait of those who'd come to accuse him: Bulger, he came to believe, may not have been an informant for the FBI but a man who received tips from the FBI.

And so in the upcoming CNN movie "Whitey," Berlinger — a pioneer of the prosecutorial-overreach subgenre with his 1996 West Memphis Three film "Paradise Lost" — makes a powerful, contrarian point about the prosecution. Even those horrified by Bulger's acts may emerge wondering whether the FBI's role in enabling his reign of terror was far bigger than coverage of the trial suggested.

"I'm not an apologist for Bulger; he's a brutal, vicious killer," Berlinger said. "But he should have had a full and meaningful defense. There are too many swirling questions of corruption [among law enforcement] that have yet to be answered, a horrendous level of malfeasance that's gone unaccounted for," referring to a group that has largely gone uncensured.

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The director has built a career on slowly uncovering prosecutorial misconduct. "Paradise Lost," a key early text in the genre, began as a look into the negative influence of heavy-metal music until Berlinger arrived in Arkansas and found a very different story, as the three young men accused of murdering boys as part of a Satanic ritual began to appear innocent.

Indeed, many of these filmmakers didn't look to become advocates. Todd Miller, director of the Sundance breakout "Dinosaur 13," which will be released to theaters by Lionsgate this year and will also air on CNN, said he wasn't sure whom to believe when he first optioned the book "Rex Appeal" by Pete Larson that recounted how Larson's discovery of the fossil of a Tyrannosaurus rex known as Sue led to a confiscation by the FBI. But extensive interviews convinced Miller that the U.S. government had overstepped its bounds.

"I didn't set out to be an activist, but if you have a compelling story it almost forces you to become one," said the Brooklyn-based Miller.