River Valley farm stands at the end of an earth road leading out of Holcomb, a small town on the western edge of Kansas. You can see its pretty white gabled roof floating above a sea of corn stubble. The house is famous for the elm trees which line the drive, giving it the tranquil air of a French country lane. The trees are in poor shape though, and desperately in need of pruning; their branches, leafless now, protrude at wild angles.

There's something else not quite right about the setting. There is a large "stop" sign at the entrance to the road, backed up by a metal barrier and a hand-written poster in red paint proclaiming: "No Trespassing. Private Drive." The warnings seem belligerent for such a peaceful spot.

The explanation for these warnings lies about half a mile away in Holcomb's local park. A memorial plaque was unveiled there two months ago in honour of the former occupants of River Valley farm: the Clutter family, who lived in that house at the end of the elm drive until one tragic night half a century ago. The plaque carries a lengthy eulogy to the family, recording the many accomplishments of the father, Herb Clutter, and telling us that the family's leisure activities included "entertaining friends, enjoying picnics in the summer and participating in school and church events".

Towards the end of the inscription it says that Herb, his wife Bonnie, and two of his four children Nancy and Kenyon, "were killed November 15 1959 by intruders who entered their home with the intent of robbery".

That is a very minimalist way of describing a multiple murder that devastated the town of Holcomb, inspired one of the great books of American 20th-century literature and spawned a stack of Hollywood films on that fateful night exactly 50 years ago this Sunday.

"Four shotgun blasts that, all told, ended six human lives." That was how Truman Capote summed up the murders with somewhat greater drama, referring to the four Clutter victims and their two attackers who died later on the gallows. After reading a short newspaper account of the killings, he decided to make the 1,700km journey from his home in New York to Holcomb to chronicle the impact of terrible violence on a small community. The result, six years later, was In Cold Blood. It propelled him to household fame and fortune, and in the process ensured that Holcomb was put on the map, and changed forever, in ways that many of the townspeople did not – and still do not – appreciate.

It is hard to think of any murder case involving six relatively unknown individuals that has captured so many imaginations. In Cold Blood has sold millions of copies and been translated into 30 languages. It was made into a black-and-white film of the same name in 1967 and there was a colour remake in 1996. The story of how it came to be written became the 2005 movie Capote, followed by Infamous the following year.

Kevin Bascue, the sheriff of Garden City, Holcomb's neighbouring town, is well used to the attention. He spreads out on his office table a set of files relating to the Clutter case, one of which records recent visitors who have come on a sort of In Cold Blood pilgrimage from Italy, Japan and all over the US. Next week Bascue will host a producer from Boston who wants to turn the book into a musical.

"The fact that someone from New York like Truman Capote felt compelled to come out all this way to tell the Clutters' story I suppose means it's forever going to be ingrained in people's minds," the sheriff says.

There are many reasons why In Cold Blood has become so ingrained. There is the precision of Capote's writing, which resonates from the first sentence: "The village of Holcomb stands on the high wheat plains of western Kansas, a lonesome area that other Kansans call 'out there'." There is the depth of his research, which he carried on to the end as a witness at the killers' hanging in 1965.

There is also something monumental about the timing of the book. America in 1959 was at a crossroads. It was still bathing in the victory of the second world war and ensuing economic boom. The country was confident and secure, and the body blows of Vietnam still lay ahead.

Nowhere was this sense of purpose more evident than in the US heartlands, with their hundreds of tight-knit communities, like Holcomb, scattered along railway lines across the Great Plains. Capote noted with satisfaction that Holcomb itself lies almost in the exact middle of the continental US.

If Holcomb was representative of that small-town rootedness that defined 1959 America, then the Clutters were representative of Holcomb. "Of all the people in all the world," Capote quotes a local detective, "the Clutters were the least likely to be murdered."

Herb Clutter was an upstanding, dependable character. As the memorial plaque notes, he was involved in the local schools, hospital, church, and was president of the National Association of Wheat Growers – a title that meant something in those days before the advent of mass-production farming.

"He was a big influence in my life – when you know somebody like that it tends to kind of inspire you," says Holcomb resident Bob Rupp. "He could see above what most people could see, and visualise how things should be."

It was Rupp's idea to erect a plaque for the Clutters. He knew the family well. So well, in fact, that In Cold Blood's first chapter heading implicitly refers to him: "The Last to see Them Alive."

Rupp was 16 at the time of the killings. His childhood sweetheart of the same age was Nancy Clutter. "She was just a beautiful, outgoing person who had many, many friends. She was just a stand-out individual," he says now.

Rupp was indeed the last person to see the Clutters alive – other than the killers – having gone over to River Valley farm on the Saturday night. "Herb was in his study, but Nancy, Kenyon and I sat and watched TV. When the news came on at 10 o'clock it was time for me to leave."

He had planned to go that night with Nancy to a midnight movie but Herb had persuaded them to go the night before. Had they gone after all, things might have been different, at least for Nancy. "You never know . . ." Rupp says, then tails off.

A few hours after he left the farm to walk home, in the early hours of Sunday, tragedy arrived at River Valley farm in the form of Perry Smith and Dick Hickock, recently discharged thieves who had driven 400 miles expressly to rob the Clutter home, expecting to find in it a safe containing $10,000.

In Capote's telling, Smith and Hickock were the other, dark side of Herb Clutter's America. They were everything he was not: impetuous, profane, rootless, lost. All Herb's striking moral strength was just as strikingly absent in them.

Sheriff Bascue's files contain the killers' confessions. Smith's statement, typewritten on faded yellow paper, is particularly chilling. "That's when I cut Mr Clutter's throat," he tells the officer leading the investigation, Alvin Dewey. "I says, 'Should I shoot him?' and Dick looked at me and he says, 'Yeah, go ahead; go ahead.' I raised the shotgun and pulled the trigger."

Hickock's statement, written in the third person, describes the shooting of Nancy. "Before Dick shot her, Nancy said, 'Oh please don't, please don't.'" Another file contains the police forensic photograph of Nancy's body. She is curled up in a foetal position, her hands tied behind her back, blood splattered on her bedroom wall.

When they left the farmhouse, leaving behind the bodies of the Clutter parents and their two younger children, Smith and Hickock took with them all the cash that Herb had on him – the princely sum of about $40.

Capote doesn't shrink from exploring the brutality of the killers, but he also forces us to consider their wounded humanity. In Perry, in particular, he captured an extraordinarily complex character, one capable of placing a pillow beneath Kenyon's head to make him more comfortable minutes before shooting him dead.

If the moral clash between Herb Clutter and his killers lies at the core of In Cold Blood, a clash of a cultural kind lay behind its writing – between Capote and the farming folk of Holcomb whom he came to live among. Capote was 5ft 4in, openly gay, with a squeaky voice and flamboyant fashion sense, as portrayed brilliantly by Philip Seymour Hoffman in his Oscar-winning role in Capote, and Toby Jones in Infamous.

Holcomb men, by contrast, dressed then pretty much as they do now. When I meet Rupp he is wearing a baseball cap, blue denim shirt and jeans, and cowboy boots. "The type of person Capote was threw me off right away," Rupp says. "He wasn't the kind of person I wanted to spend time with – he was very, very strange."

Nelle Harper Lee, Capote's friend and fellow writer who came with him to Holcomb, once said: "Those people had never seen anyone like Truman – he was like someone coming off the moon."

Capote's first few days in town were difficult. Everyone gave him the brush-off. Harper Lee was crucial in overcoming the initial hostility. She had just finished writing To Kill a Mockingbird, and was awaiting publication. She agreed to accompany Capote to act as what he called his "assistant researchist". Her affable southern manner – she and Capote were childhood friends from Monroeville, Alabama – made for a much happier connection with the plain-spoken inhabitants of Holcomb. Rupp recalls that in the one interview he granted Capote, most of the questions were asked by Harper Lee, so much so that "sometimes I wonder who really wrote that book".

Delores Hope, who in 1959 was a columnist for the local paper, the Garden City Telegram, also noted Harper Lee's vital role. "Nelle sort of managed Truman, acting as his guardian or mother. She broke the ice for him."

Delores and her husband, Cliff Hope, were also crucial ice-breakers. Hope was the Clutters' family lawyer, and arranged for Capote to look around River Valley farm so he could describe firsthand the murder scene. He is one of the few people acknowledged by Capote at the start of the book.

As Christmas approached in 1959, Delores took pity on Capote and Harper Lee, imagining them sitting in their hotel rooms with nowhere to go. So she invited them to Christmas lunch. "We asked them to come over at one o'clock," recalls Cliff. "Truman said: 'We'll be there at two.'"

Capote arrived at the Hopes' doorstep clutching a bottle of J&B whisky for his own consumption, and from the minute he stepped indoors, as Delores puts it, "he pretty well took over the conversation".

While Capote held forth, Harper Lee accompanied Delores into the kitchen, where they bonded over cooking a goose. They became great friends, and remain in touch to this day, despite Harper Lee's reputation, at the age of 83, for being reclusive.

The Hopes still live in the same house where they welcomed the odd couple. The simple wooden home, with its postwar furniture, has changed remarkably little, as if stuck in 1959. As they describe Capote's effervescent monologue, you can almost see him sitting on the beige cloth couch, legs crossed, waving his arms around as if starring in his own movie.

With the help of the Hopes, Capote and Harper Lee went on to inveigle themselves into the lives of other key figures in town, notably the Deweys with whom Capote became lifelong friends. He and Harper Lee were at the Deweys' house on the night when news came through that the killers had been arrested in Las Vegas. The extent to which Capote won over key figures of the community was such that when he left to return to New York in January 1960 after his first tranche of research, he told a friend: "At first it was hard. But now I'm practically the mayor!"

Yet there are few visible relics of of Capote's time in Kansas. In Holcomb, there is River Valley farm that still looks on the exterior largely as he described it. It now belongs to its third set of owners since the Clutters, the Maders; they used to give tours of the property but grew so bothered by the endless stream of In Cold Blood pilgrims that they posted the "stop" sign.

In Garden City, the Wheat Lands motel where Capote and Harper Lee stayed, is still there, though a photo of Capote posing in front of the building has been stolen from the foyer. The courthouse where Smith and Hickock were put on trial still stands as imposing as it was then. In the cemetery there are three neat tombstones, all bearing the date 1959: Herb and Bonnie together in the centre, Kenyon on the right and Nancy to the left. Someone has left a vase of blue cloth flowers; it has tumbled over.

Those signs apart, the local community is barely recognisable 50 years on. The family farm as the prime social unit, of which the Clutters' was the epitome, has declined and given way to huge mechanised operations producing animal feed. Holcomb, population 270 in 1959, has grown tenfold and is now dominated by one of the world's largest meat-packing factories. It is the last sad irony of Herb Clutter that just a few years after his own violent death, his way of life died too.

For Truman Capote the outcome of his sojourn in west Kansas was decidedly mixed. In Cold Blood, which he immodestly heralded as a new form of non-fiction novel, was received with delirious approval; Norman Mailer dubbed Perry as one of the great characters in American literature. The book earned its author more than $2m, which he used to buy homes in Manhattan, the Hamptons, Palm Springs and Switzerland. But by all accounts such heavenly success also went to his head, and contributed to his downward spiral in a haze of lavish parties, drink and drugs. He failed to write another substantial work, and died in 1984.

In Holcomb and Garden City, some of the residents welcomed his book. Alvin Dewey, the chief police investigator, championed it to the end. The Hopes too remain fans, cherishing the first-edition copy that Capote autographed for them. But many in the town continue to resent its intrusion, and refuse to talk about it or any of the subsequent films. Cliff Hope puts the ongoing hostility down to Capote's unblinking portrayal of the killers. "Many people thought he should have written about the Clutter family, rather than the murderers."

Delores's theory is that some local people have closed minds. "There will always be people who think it's none of anybody's business to come out here and write about their affairs. You will never change their opinions."

Bob Rupp has a third view. He says he has never read In Cold Blood, nor seen the movies, and never will. But he believes that Capote was unfair to the Clutters, because he left to posterity a memory of them that is dominated by the gruesome manner of their deaths rather than the wonderful accomplishments of their lives. He still thinks about the Clutters often, hence his idea for the memorial. But he has moved forward. He married in 1963 and now has four children and eight grandchildren.

So does he think that Capote's fateful arrival in Holcomb all those years ago has in the long run damaged the town? "I don't know it's really been damaging. I don't think he did the Clutter family justice, is all."

Dewey, who nailed the killers and became Capote's good friend, was asked the same question before he died in 1987. "What happened, happened," was his answer. "Four good people were murdered. An author came and wrote a book about it. In all communities, things happen. Good and bad. Those are the facts."