The film-maker Jeremy Marre's harsh documentary on life, death and other business insidetheUSprisonsystemhasbeen renamed for its Channel 4 broadcast, and the new title sums things up with inarguable, Ronseal-ish efficiency: America's Deadliest Prison Gang. But preview copies come bearing Marre's original title, and it sets the nightmare tone more forcibly: Reigning In Hell.

It's probably best to do without the Milton allusion. There's nothing poetic about Marre's blunt film, which lays its story out with a commendable lack of style. The gang under observation is The Aryan Brotherhood, the most notorious of America's prison fraternities. Formed amid the racial tensions that erupted with the desegregation of America's prisons in the early 1960s, the all-white group has grown to conduct a decades-long reign of terror behind bars, controlling things where the guards leave off, and in many instances controlling the guards, too.

From its earliest days, the Brotherhood sought to set itself apart from the other gangs not through strength of numbers, but by its selectivity. Quality not quantity of membership is the key, although in this case quality comes measured in degrees of psychotic depravity. Members make up less than 1% of America's prison population, but are believed to be responsible for 26% of prison violence.

Theirfavouredmethodofcontrolis brutal murder, with multiple stabbingthe favoured method. "Bring extreme violence to a situation one or two times," one member explains in the chirpy tones of a motivational speaker, "you don't have to do it no more - then you can get down to business!"

Marre litters his film with hellish prison CCTV footage depicting Brotherhood members attempting to stab other prisoners to death; grainy, ungainly scenes of blind, flailing viciousness, panic and death. The shots feel vital in hammering the grim reality home, but their inclusion is no less troubling.

More extraordinary yet is the unprecedented access Marre has gained to the gang. With its "blood in, blood out" vow of silence, associates have not traditionally been keen to court the media. Some are locked down in high-security isolation. Some are in a different cage, old men now, staring out from their skin with the same dead eyes you see in the mugshots of their youth.

Then again, it's not surprising these men would be keen to talk about themselves. As its name suggests, the organisation dallies with self-aggrandising delusions of purity and elitism. Members are actually assigned reading lists - Plato, Nietzsche, Sun Tzu; the Fight Club book group.

"Organisation" is the crucial word. By the mid-1980s, the Brotherhood's power had mushroomed into a lucrative network of murder and drug-dealing that spread across America, not only within the prison system, but out on the streets as well. It was finally cracked in 2005, when Federal prosecutor George Jessner had the idea of going after it armed with the Rico Act, the laws drafted to deal with organised crime in the States. "It's the most murderous gang in America," Jessner says. "They killed more than the Mafia." Marre's film opens with the bizarre sight of police actually launching a dawn raid on a prison, rounding up suspects from cells.

Despite his revulsion, Jessner retains a grudging note of respect when discussing the Brotherhood. Along with the all-too believableviolence,thefilmuncovers incredible details, including how the group communicated via secret codes based on Sir Francis Bacon's 17th century ciphers. Marvelling at that warped ingenuity, investigators speak of talent wasted. "These are guys that would have been leaders in whatever field they went into," one says. "If they weren't behind bars, they'd be running America." The more you think about it, the more unsettling that statement gets.

From this concrete hell, it's a relief to turn to the rain-cooled panoramas and almost genteel discussion of murder that makes up Ian Rankin's Hidden Edinburgh. Made to mark the writing of the final Rebus book, this very watchable documentary follows the author on a tour of the shadowy corners of the city that has inspired him, as he ponders how to complete his long goodbye tothedetective.Rankinrakesdown through the layers of crime and killing, recentandancient,thatseemsoaked through the bedrock of the place, picking over the bones with an almost professorial mix of insight and baffled quizzicalness. It's manna for Rebus fans, even if it draws them nearer to The End.