Director of Presumed Guilty, which exposed miscarriage of justice in Mexico City, awaits rulings on claims for compensation

When Presumed Guilty – the nightmarish tale of a street vendor in Mexico City who was twice wrongly convicted of murder – was released in 2011, it won an Emmy for investigative journalism, broke Mexican box-office records, and triggered a fierce debate over the country's dysfunctional judicial system.

Nearly three years later, as important legal rulings are expected in two cases over the documentary, the film is banned in Mexico and the director has received death threats.

"The caller told me to tone things down or else," said Roberto Hernández. "The first threat was veiled, the second directly said they would kill me."

Hernández and his team set out to highlight the number of innocent people in the capital's jails through the story of a street vendor convicted of murder on very flimsy evidence amid obvious abuses of authority and due process. Less than a month after it was released in Mexico, the film was banned, but it became a pirate DVD sensation, raising hopes that the country's courts might be forced to clean up their act.

Those hopes soon turned into fears of a clampdown on freedom of expression as a barrage of court cases against the film gathered steam. The final court hearings in one of the damages cases was held last week, with a ruling expected in a couple of months. Taken together, the compensation sought hovers around 3bn pesos (£150m).

"They are trying to make us litigate until we bleed," Hernández told the Guardian. "It is a terrible message for other journalists who might want to follow in our steps."

The film's legal problems mostly stem from complaints from a witness who admits, on camera, that he lied about seeing the street vendor, Antonio Zuñiga, shoot the murder victim, and from the police officer who arrested Zuñiga without a warrant or any obvious grounds for suspicion.

Neither plaintiff disputes the accuracy of the documentary, arguing instead that they never agreed to appear in it and have since been insulted in public places.

Meanwhile, the ban on showing, broadcasting or selling the film in Mexico appears to be on the point of being revoked – but only because the judge ruled the complainant had not provided "anthropometric proof" that he actually appears in the film.

"It is the stupidest thing," Hernandez said. "What we need from the court is an argument about freedom of expression."

The film-maker has directly accused the Mexico City's judicial hierarchy of secretly driving the cases against Presumed Guilty forward in an effort to exact vengeance for the way the film exposed the entire system.

Edgar Elías Azar, the city's most senior judge, has denied any kind of meddling. "We cannot shut the door to these people who are looking for compensation for the exploitation of their story, their life and their image," he told reporters last week.