On the evening of Sunday 2 November 1997, in the hours before her death, Robin Hall waited for the mail. Robin, who was 33, lived on the outskirts of Camden, New Jersey, the post-industrial city where she had grown up. Her home, a one-bedroom garden unit that her younger sister Tracey was renting for her, was largely empty – she owned no fridge and little furniture. Lately, Robin had borrowed money from friends. She had assured them that when her social security cheque arrived, on Monday, all debts would be paid.

Robin was living in a complex called the Ferry Station Apartments: a handful of pale brick buildings capped by low, shingled roofs, with brief lawns of crabgrass and mangy shrubs, set beside the unkempt grounds of the New Camden Cemetery. The development stood a few blocks from Tracey’s home, and Tracey, a police officer, had been keeping close tabs on Robin. Despite its stark appearance, Ferry Station seemed to her a vast improvement over the drug-wracked neighbourhood where her sister had holed up that August, while their mother was dying from cancer.

Tracey had tussled with Robin before over her choices. In 1991, their brother, Troy, had been killed by a drunk driver. Grief-stricken, Robin had descended into drug abuse. At the time, crack cocaine was going for about $2.50 a vial in Camden, which routinely ranks among the poorest cities in the United States. Tracey began hearing about Robin from other cops, who often spotted her in tumbledown buildings known as the Taj Mahal and the Doghouse, popular haunts of drug users.

Since then, Robin had gone to rehab, but she struggled to stay clean. Though she had a head for figures, it had been years since she minded a teller’s window at any of the local banks where she once worked, and longer still since she dealt blackjack in Atlantic City. It had been there, after a stint at college, that she met the father of her son, Amadeus, who had just turned nine. Now, her mother’s illness had sent her into a tailspin.

That Sunday evening, Robin went to see a friend named Stan Courtney, a 60-year-old retiree, who also lived at Ferry Station. The two of them talked about Robin’s boyfriend, whom she had visited that day at a local jail. He was due out in December, and she was excited for the homecoming. Robin and Courtney made plans to meet the next day. After their cheques came, they would go grocery shopping.

But Robin never showed up. On Tuesday afternoon, Courtney still hadn’t heard from her. “So I said, ‘Damn, let me go check on Robin,’” he later told a detective.

As he approached her front door, he felt a premonition. “The feeling’s so strong,” he remembered. “I don’t know why.” Robin’s door stood several inches ajar. In her bedroom, blood was spattered on the wall. “Just looking at her, I knew that she had deceased,” Courtney said. “I mean she was dead, no doubt in my mind.” Robin lay prone, naked from the waist down, with her torso on the bed and legs draped below, like a climber trying to shimmy on to a ledge. Her head had been staved in.

During the homicide investigation that followed, detectives failed to identify a suspect, and in subsequent years, none would emerge. For several days after Robin’s body was found, Tracey struggled to think of a reason why someone might murder her sister. If Robin had been in debt, the lender could have come to Tracey for money. And though Robin had her faults, she had never been a troublemaker. If anything, she had fallen too easily under the influence of others.

After Robin was buried, Tracey tried to put her speculation to rest. She had three daughters to look after, plus Amadeus, who came to live with them. In time, the cousins would call one another brother and sister, and Amadeus would call Tracey his mother. When he was asked where his real mother was, he would say that she had been murdered, and that the crime had never been solved. But it seldom came up. Missing relatives were common among his classmates.

Just looking at her, I knew that she had deceased. I mean she was dead, no doubt in my mind Stan Courtney

Amadeus finished high school near the top of his class, which was small, because many students didn’t graduate at all. He won a football scholarship to a nearby college. At 6ft 2in tall, and a muscular 245lb, he was swift and broad-chested, a tenacious pursuer of quarterbacks. He wore dreadlocks in the fashion of Lil Wayne, whom he idolised, and had a wide, beautiful smile.

In 2012, Amadeus, who was 23, was considering both law school and professional football. But since becoming a teenager, he had twice attempted suicide. That May, on the morning after one of his sisters’ proms, he declined to drive to the seaside with his family. Instead, he stayed in his room and composed a farewell note on his iPhone. Then he retrieved Tracey’s service weapon from her bedside drawer and shot himself in the head.

The apartment block in Camden, New Jersey, where Robin Hall was murdered. Photograph: Joshua Bright/The Guardian

Early in 2015, Tracey got a call from Terry King, a lieutenant in the homicide division of the county prosecutor’s office. Scott Thomson, Camden’s police chief, had recently assembled a cold case squad, dedicated to re-examining unsolved homicides. King asked whether Tracey would be willing to meet with the unit to discuss her sister’s case.

Robin had been one of 45 people murdered in Camden in 1997, an unremarkable year in a city that has averaged more than 40 killings annually over the last quarter century. The fact her murder had not been solved was hardly more unusual. In 1988, charges were filed in some 93% of Camden homicides. Since then, the rate at which murders are solved has declined precipitously, hitting 33% in 2014.

Across the US, more than 210,000 homicides committed since 1980 remain unsolved, with many cold cases concentrated in poor, predominantly black cities such as Camden, Detroit, and New Orleans. Thomson had for some time been troubled by the trend. But local history – both recent and long in the past – had made it difficult for him to address the issue. The last 60 years had left Camden largely disconnected, legally and financially, from the rest of the state. And the dictates of the violent informal economy that had replaced the city’s once-thriving industrial life made solving new crimes difficult, let alone old ones.

Camden sits on the east bank of the Delaware River, just across the water from Philadelphia. During the second world war, it housed a shipyard that employed 30,000 people. Franklin Hall, Robin’s father, grew up not far from it. His own father worked there. Robin’s mother, Augusta, spent her childhood in Camden, too. Both families had come north as a part of the great migration, the movement of roughly 6 million African Americans from the rural south into northern cities during the middle of the 20th century. In addition to shipyard work, Camden, which was then inhabited largely by Jewish, Polish, Irish, and German immigrants, held the promise of jobs in factories making tin cans, cigars, radio parts, and other items.

Between 1960 and 1970, however, Camden’s population fell by 15,000. As in other cities across the country, white residents fled to the suburbs amid racial tensions. (Camden is the capital of Camden County, a division comprising 37 majority-white suburbs, with a patchwork of affluent and working-class communities.) More than 17,000 jobs disappeared. The shipyard went bankrupt and factories were shut down. Urban renewal initiatives, often left unfinished, shunted thousands of minority residents into increasingly untenable living conditions. In 1973, Angelo Errichetti – the first of three Camden mayors to be jailed for corruption between then and 2000 – described the route from his home to City Hall: “It looked like the Vietcong bombed us to get even.”

By the new millennium, Camden faced some of the worst violence in the US. The situation further deteriorated between 2011 and 2013, when the police department – which had long been hobbled by misconduct and incompetence – lost about half its staff to budget cuts. Police officers largely stopped responding to non-violent crimes. Drug dealers, who then operated some 175 open-air markets in Camden, donned T-shirts bearing the slogan, “It’s our time”. In 2012, the city of 78,000 tallied 67 murders, making it, per capita, the most dangerous in America.

“We have, like many urban centres, struggled with our murder clearance rate,” Thomson acknowledged last year, when I met him in his office. Under ideal circumstances, a homicide detective might address five or fewer cases per year; between 2011 and 2013, investigators in the homicide division of the Camden County prosecutor’s office handled perhaps 30 or 35 murders each. “With the resources available to us, it was hard to keep up,” Thomson said.

Robin Hall’s graduation photo. Photograph: Joshua Bright/The Guardian

Thomson is a lean, dark-haired man with warm, formal manners and rigidly upright posture. Since the layoffs, the department has been rebuilt with new hires. Thomson has emphasised community policing; dispatching beat cops to talk informally with residents and business owners, in an effort to nurture relationships in “non-enforcement” situations. In 2014, the number of murders fell to 33, and last year, to 32.

Still, Thomson knew that there were dozens of families in Camden like the Halls, most of them living with unresolved anger and grief. How could he expect to cultivate trust when so many crimes had gone unprosecuted?

In the first months of 2014, as the city’s violence ebbed, Thomson began putting together his cold case unit. Other cities around the country had squads devoted to long‑unsolved homicides, but the logistics and expense involved had made such a unit unfeasible in Camden. Thomson’s was to be the first cold case team in the city’s history.

In 2012, the city of 78,000 tallied 67 murders, making it, per capita, the most ­dangerous city in America

From the start, he knew that it would annoy some of his subordinates: the squad’s existence might seem to imply that some homicide investigations had been botched in the first place. “It’s always a challenge when you create this kind of unit,” Thomson told me. “The very idea of it goes against the culture of police departments; you have to be careful to let folks know that it’s not about critiquing other people’s work.”

One of Thomson’s first calls was to Marty Devlin, an investigator who had retired four years earlier, at 65, from the Camden County prosecutor’s office, in accordance with departmental policy. Thomson regarded Devlin as a natural first pick for the unit. Devlin had spent 28 years as a detective in Philadelphia before moving, in the early 1990s, to the prosecutor’s office. There, he had achieved minor fame as the lead investigator in the successful murder prosecution of a prominent rabbi in the affluent suburb of Cherry Hill.

A jovial man, with a black belt in taekwondo and a penchant for tweed blazers, Devlin sees himself as an inhabitant of the wrong era. “I would have liked to be born in the time of marshals and sheriffs,” he once told me earnestly. “I truly love keeping the wolf from the lambs.” Over the course of his career, Devlin has investigated more than 1,000 homicides. With characteristic self-confidence, he told me that he guessed he had solved more than 90% of them. Joe Forte, 60, another veteran investigator drafted out of retirement for the cold-case unit, worked with Devlin at the prosecutor’s office. “When I first met Marty, I didn’t know him from a can of paint,” he told me. “But if you ever happened to go to Philly on a case, people would talk about him like he was a god.”

The cold case team, which in addition to Devlin and Forte includes just two other detectives, accords priority to cases with “meat on the bone” – those in which the first investigators left behind substantive material to work from – rather than killings that are the oldest or most horrific. Resources remain thin, and the county needs results to justify the unit.

Not infrequently, it is the presence of DNA evidence that suggests that a murder file might warrant fresh examination, particularly in cases more than a few years old, for which genetic evidence has never been subjected to modern analysis.

During the original investigation of Robin Hall’s murder, male DNA had been recovered from Robin’s body, but no match had ever been found. It seemed like a good case for Devlin’s team. Initially, though, when the prosecutor’s office approached her, Tracey was apprehensive. She had lived for so long without resolution. Maybe, she thought, it would be better to let things be.

I would have liked to be born in the time of marshals and sheriffs. I truly love keeping the wolf from the lambs Marty Devlin

Eventually, she came around. For reasons that she could not quite articulate, she felt more hopeful about finding answers this time. She knew that most Camden murders were not true mysteries. So-called secrets were often widely known – kept from authorities because of fear, disdain, or adherence to a code of non-cooperation. The people with whom Robin had associated during her last months had lived hard lives. Perhaps, she thought, they had re-evaluated their priorities, shifted their allegiances. Memories once addled by drugs might have grown clearer, and maybe the truth would finally be allowed to surface. Everyone has a conscience, Tracey thought. She had to believe that was true.

For a homicide detective, determining precisely when a victim died is essential. It can confirm the alibis of some suspects, while raising suspicions about others. The county medical examiner’s report on Robin Hall’s murder, which first appeared on Devlin’s desk last summer, estimated that she had died six to 12 hours before the examiner’s arrival on the scene, at 7.30pm that Tuesday in November 1997.

But Devlin knew that time-of-death estimates could be problematic. Three processes are of primary importance to time-of-death calculations: the progress of rigor mortis, the cooling of the body, and the settling and pooling of blood that begins when the heart stops pumping. All three are affected by ambient conditions – temperature, airflow, moisture – and their exact influence can be difficult to judge. Devlin recalled a mantra he had often heard from Dr Robert Segal, a longtime Camden County medical examiner: “We may say that death occurred at some point between the time the victim was last seen alive, and the time the body was found.”

Modern jurors often assume that they will be presented with the kinds of evidence they have seen on television: DNA analysis that definitively condemns or exonerates the accused; expert testimony from blood-spatter specialists; accurately reconstructed crime scenes. But the nature of Camden’s violence often means that the crime lab has little to tell detectives. In 2015, gunshots accounted for all but one of the city’s homicides; most of the shootings occurred outdoors, where a killer’s hair, bodily fluids, or fingerprints are unlikely to be recovered. Guns also make physical contact between perpetrator and victim unnecessary, reducing the odds that evidence gets transferred from one to the other. With their expectations frustrated, jurors sometimes grow dubious about the credibility of the prosecution, a tendency that helps make Camden homicides acutely difficult to try. “Science is a great tool,” Devlin told me. “But it needs to be put in its proper context.”

In this case, Devlin noticed that the first medic on the scene had found Robin’s body cold. Yet the medical examiner later judged it warm. Later still, a crime lab technician diagnosed a degree of rigor mortis too advanced, in Devlin’s view, to be consistent with the time-of-death estimate.

With forensic science sending mixed signals, Devlin fell back on what he knew about Robin’s lifestyle. From the statements of Stan Courtney and others, Devlin was aware that Robin had been counting down to the arrival of her social security cheque. Yet when investigators had entered her apartment, the cheque lay in an envelope near the front door. It had been slipped through the mail slot around 12.30 on Monday afternoon. Devlin could not fathom a scenario in which Robin would have failed to collect it. “Junkies are expecting that cheque,” he said. “When it gets there, they’re going to be dressed and ready to go. If that cheque hits the floor and she don’t touch it, she’s already dead.”

If true, that meant a substantial change in the timeline. The killing would have had to occur between Sunday evening, when Robin left Courtney’s apartment, and 12.30pm the next day. The frame for the crime had shifted by more than a day.

Marty Devlin and Joe Forte at work in their Camden County Police Department office. Photograph: Joshua Bright/The Guardian

Devlin and Forte share their windowless office, on the second floor of police headquarters, with Peter Longo, a 41-year-old investigator from the prosecutor’s office, and Shawn Donlon, 46, who was plucked for the cold case unit from the police department. The room is hung with city maps stickered to show the locations of murders in recent years, and with a whiteboard devoted in part to tracking Devlin’s taste for ill‑defined neologisms (“prosidulator”; “put a boogey on ’em”), non sequiturs (“That’s because of Madonna!”), and hard-boiled koans (“There’s no such thing as a secret”; “Time is the enemy of love and homicide”).

Situated off the central detective’s bureau, the room contains four metal laminate-top desks. Rust stains bleed through the ceiling tiles, and blue and yellow ethernet cords snake over mangy grey carpeting. The mood tends to be lighthearted, reminiscent of a bus ride with a college football team, although Forte often injects a little gallows humour into the conversation. The investigators dress in plain clothes: Forte and Devlin in blazers and slacks, Longo and Donlon in dark khakis and cotton button-downs. The lone liberal, Forte, who is bald and mustachioed, takes flak for watching tthe Rachel Maddow Show. Donlon and Longo are hulking, gentle, soft-spoken men who generally pair off for lunch, a meal to which they devote a lot of thoughtful planning. Forte likes to lampoon their indulgences: “Shawn and Pete, for them every day’s Thanksgiving!” Donlon and Longo, in turn, call the older men “the dinosaurs”, for their spotty understanding of technology and pop culture.

No strict parameters dictate that a case has gone “cold”, but of course time – months, years, decades – is the common element. One idiosyncrasy of investigating old murders is that the slow pace of the work can seem at odds with its underlying urgency. The frantic door-to-door canvassing that characterises “live” homicide investigations, and which can keep detectives awake for days, has little place in cold cases. There is, however, a lot to read: clinical reports, detectives’ summaries, interview transcripts. Audio and videotapes demand playback, and crime scene photographs careful review. Devlin colour-codes documents with fluorescent highlighters, while Forte makes longhand digests on legal pads. (Donlon and Longo prefer computers.) The job to some extent resembles that of a historian, who might work through substantial archival caches before interviewing living sources.

The passage of time can give cold case investigators advantages over detectives responding to new murders. When Georgianna Jedrzejewski, a homeless woman, was killed in Camden, at the corner of 8th and Tulip Streets, in December 2012, the neighbourhood was controlled by a drug dealer. Less than a year later, he and a group of associates were arrested for drug trafficking. When Jedrzejewski’s case was reopened soon thereafter, previously uncooperative witnesses, who no longer feared reprisals, began talking. Two witnesses claimed that a man associated with the drug dealers was the murderer. He was subsequently apprehended.

In the two years since the unit began operating, Devlin and his team have cracked four cold cases, including one from as far back as 2002. For the families of victims, the effect has often been cathartic, if joyless. But there is still a long way to go in repairing the local reputation of Camden’s police force. According to the Philadelphia Inquirer, in 2014 Camden had the highest number of excessive-force complaints in New Jersey – even compared to larger cities such as Newark and Atlantic City.

Father Jeff Putthoff is a Jesuit pastor and the founder of Hopeworks ’N Camden, a non-profit organisation dedicated to job training, youth work and trauma counselling. “Camden is a city of tremendous grief, over many years,” Putthoff told me. “Addressing [unsolved killings] is absolutely emotionally meaningful for Camden. But in some ways the unsolved murders are just the tip of the iceberg. What we have is a triaged response to an epidemic proportion of injury: stop the killings, solve the killings. It’s a little like saying that it was just the top of that iceberg that sunk the Titanic. Well, no. It was actually all the stuff underneath.”

The intersection of Tulip Street and South 8th Street where Georgianna Jedrzejewski was killed in 2012. Photograph: Joshua Bright/The Guardian

In the year after Robin Hall’s murder, investigators interviewed roughly 30 people in connection with the crime. Only a handful, with whom Robin spent many hours during her last weeks, held real interest for the cold case detectives. There was a man named Gus, who had lived across the street from Robin, and a woman named Genique, with whom Robin had become close as her addiction deepened. There was Kennedy Williams, and a man who everyone called Rogie. There was a particularly desperate man, who lived alternately in an abandoned building and in a makeshift shelter in one of the city’s cemeteries. Robin had often let him shower at her apartment.

Taken together, their statements depicted a kind of narcotic micro-society. Its members loaned one another cigarettes and money, and pooled resources for drugs. It wouldn’t have been unusual for them to come or go from the Ferry Station Apartments at any hour. But who killed Robin Hall? Well, they said, it could have been anyone.

Of course, that was not quite true. The evidence indicated that Robin knew her attacker well. Her door showed no signs of forced entry, and the crime scene suggested a viciousness unlikely to arise without some heated personal dispute. Robin’s body had been arranged in a humiliated posture, and bore evidence of sexual abuse. The apparent murder weapon, made from a tube sock slung with a rod-shaped weight, was found bloodied at the scene. Had he not been imprisoned at the time of the murder, Robin’s boyfriend would have made a natural suspect. Interviews suggested, however, that Robin had sometimes traded sex for drugs with male friends. This enlarged the pool of suspects.

On the Tuesday afternoon when Stan Courtney went to check on Robin, he brought with him a spare key that she had kept at his apartment. That the front door was already open struck him as unusual. To the cold case detectives, it seemed even more so in light of something else in Courtney’s statement. Courtney said that several of Robin’s friends had stopped by the night before. They told him that they had knocked on Robin’s door and received no answer. Courtney did not identify who had visited him, but a pattern of activity had been established at Robin’s home.

It wasn’t difficult to guess at the group’s makeup. It would have been useful, though, to know who hadn’t come to Courtney’s door. If Devlin was right, and the murder had taken place between Sunday evening and midday Monday, it was plausible that after Robin’s friends knocked, the next person to exit her home, leaving the door open for Courtney to find, was the killer – that he had been inside all along. The alibi of any member of Robin’s circle who had not appeared at Courtney’s doorstep would thus bear special scrutiny. Of course, Devlin told me, the scene remained hazy. And by the time he opened Robin’s file, it had become difficult to clarify. Courtney had been dead for 13 years.

During the last months of 2015, the detectives sought out Robin’s old friends. Gus, who no longer lives in the area, had attracted significant investigative interest after the killing. A sock that appeared to match the one used in the murder had been found in his home. The resemblance had ultimately been deemed superficial, however, and a DNA test failed to link its owner to the scene.

Devlin and Forte do not consider this proof of innocence, but when they called Gus not long ago, he spoke to them willingly, and in detail. The year before Robin’s death had been difficult, he told them. He had lost his job and his marriage had collapsed. To cope, he had started smoking crack. He had since got clean and returned to work. Gus remembered something important from his days hanging out in Robin’s apartment: he had seen the murder weapon before it was used in the killing. Robin had assembled it herself, he said, for protection.

Boxes of confiscated handguns in the gun vault of Camden County police department. In 2015, gunshots accounted for all but one of the city’s homicides Photograph: Joshua Bright/The Guardian

Theories of motive tend to have limited importance in solving homicides in Camden. What might seem to be a minor incident often later turns out to have produced dire consequences. In Robin’s circle, where needs were urgent and minds often fogged, violence might have erupted suddenly. As they continued their investigation, Devlin and Forte turned their attention to Kennedy Williams, who was the only one of Robin’s friends who had not cooperated with the original investigation.

The detectives believed it would be a mistake to approach Williams without leverage – and the genetic evidence from the crime scene was missing. After a storage facility beneath City Hall flooded, evidence in several older cases that had been kept there disappeared. The cold case investigators had nothing to which they could compare Williams’s DNA.

They hoped that the other members of Robin’s clique would be able to tell them what the vanished evidence could not. Finding them, though, proved difficult. Credit card bills, lease agreements, long-term phone numbers – the web of records that locates most adults within the context of modern life – did not seem to exist for many of Robin’s former friends. The disappearance of Genique, who had not so much as a driving licence on file, represented an especially troublesome lacuna: her original statement had complicated what Stan Courtney told police. Genique claimed that she had been the one to find Robin’s body. Instead of alerting the cops, she said, she had called some of Robin’s other friends, including, in all likelihood, Courtney. (Courtney ultimately called the police.)

Did that mean that Genique, rather than the killer, had left Robin’s door open? And if she had called Courtney, why would he have neglected to mention it to investigators? Devlin and Forte had little success tracking down the handful of people who might have been able to provide clarification. They, too, mostly seemed to have dropped off the grid. Other than Gus, Rogie was the sole exception. Through an ex-wife, the detectives obtained his phone number. But that breakthrough yielded just one conversation before Rogie stopped taking calls. Soon thereafter, they realised that he had given them an invalid address.

It is not uncommon for once-cooperative interviewees, like Rogie, to reconsider. Even when witnesses do agree to testify in court, Camden cannot afford to offer them protection. The best the city can do is to provide a small stipend to cover moving costs and first month’s rent. It is telling, though, that witnesses frequently refuse to leave the city. “We’re often talking about people who are destitute, and who rely on a network of friends and loved ones,” Scott Thomson, the chief of police, told me. “It’s very hard for them to survive outside that network.”

In the office one afternoon, I asked Devlin how investigations in Camden compared with those in the suburbs. Devlin was emphatic: suburbanites were more likely to cooperate with police simply because they had a stake in the society the legal system represents. “If I give you a grand jury subpoena, you’re damn sure as shit going to the grand jury!” he said. “The same is true for anyone who has a job, a business – something to lose. People in the suburbs are more likely to talk [to police] simply to avoid going to the grand jury. And when they do go, they are less apt to lie, because technically, they can be arrested for that, and they’re scared shitless.” In the last 25 years, the county suburbs, which house more than five-and-a-half times the population of Camden, have averaged just slightly more than nine murders annually – about 32 fewer than Camden. Some 84% have been solved.

One morning not long ago, I met Tracey at the Camden County prosecutor’s office, which occupies a grey concrete building separated from City Hall by a small green park. Gathered there were several dozen addicts, known locally as the Methadonians for their patronage of the clinic that stands nearby. In a conference room at the rear of the building, Tracey and I sat opposite each other at a long table. Around her left wrist, she has a tattoo that reads, All Love, with Amadeus’s football number, 85, dangling from the end like a charm.

Soon after Amadeus killed himself, Tracey had quit the police force. Her daughters welcomed the decision. It had been difficult, watching her put on her boots every day, while their family had seen no justice. Recently, her youngest daughter, Phenaysza, who was close to Amadeus, has suffered from depression. “This is one of them skeletons that’s sitting up in our closet,” Tracey told me of Robin’s murder. “And I think it’s making everybody sick right about now.”

Tracey, the sister of Robin Hall, at home. Over her left shoulder is a photo of Robin Hall’s son, Amadeus, who took his own life in 2012. Photograph: Joshua Bright/The Guardian

Even today, Tracey feels responsible for Amadeus’s suicide. She had been too insistent that they move on after Robin’s death, she said. At first, she had even let Amadeus believe that Robin died of cancer. But how do you tell a nine-year-old that his mother was beaten to death, and that no one knows why?

Nonetheless, Tracey, who now works in security, had raised four children in Camden. All had gone to public school and on to college, a fact she relates with obvious pride. Tracey is an advocate for suicide prevention, and speaks to families affected by street violence. Her neighbours look to her as a model of what is possible in their city.

Tracey remembered that when she, Robin, and their brother, Troy, were children, their mother made sure that they never missed a Sunday at St Bartholomew’s, on Kaighn Avenue. The church had been established by black Catholics in 1947, when they were unwelcome elsewhere. She remembered her parents’ three-bedroom terraced home, in a part of the city that was quiet then. “Everyone looked out for everyone else’s children,” Tracey said of the neighbourhood. “People still sat out on their porches all night.” In the evenings, her father, Franklin, a kind, serious man who worked as an engineer at a phone company, had liked to read the papers, as televised news played in the background.

Tracey remembered speaking to her father just before he died, about two weeks after Amadeus’s suicide. Tracey was told that the cause of her father’s death was cardiac arrest, but that didn’t seem quite right to her. Franklin Hall was a big, healthy, darkly complected man – 6ft 5in tall and more than 300lb, with size 17 shoes. At the end, he had faded pale.

Franklin believed family was important. As a young couple, his parents had migrated from South Carolina and settled near the Camden waterfront. His wife, who was known as Gussie, had been the oldest of 16 children. He himself had seven siblings, and soon after they got together, he and Gussie married and had children of their own. Robin came first, followed by Troy, and then Tracey. It was a period of rapid change in Camden. When the city deteriorated, Franklin moved the family to the suburbs, into a large house with an expansive lawn. Troy became a marine and Tracey a cop. Franklin had a grandson, Amadeus, who could run like the wind. But somehow, things didn’t work out. Somewhere, something had gone wrong.

“It’s just too much,” he told Tracey. “I buried everyone.”

In the last few months, I’ve called Devlin and Forte several times to check in. Though other investigations had progressed, they had nothing new to report about Robin’s case. This did not surprise them. As in countless Camden murder investigations, many of the footholds that the case initially seemed to offer – genetic evidence, several parties likely to have knowledge of the crime – had crumbled. Others might appear, but the detectives have no immediate reason to predict that they will. To proceed in their work as they do, with the assumption that every case can be solved, requires a certain frank indomitability – an existential fortitude.

Devlin once told me that on the last day before his retirement, on New Year’s Eve 2009, he worked until midnight. “I felt like I had more to give,” he said. “When that beeper went off at 2am in February and it was snowing and you had to go to River Road in Camden and there were no witnesses – that always made perfect sense to me. Some guys hated it. I never understood that.” Once cases are over, however, Devlin prefers not to think about them. “Every one I don’t remember is a blessing to me,” he said. “Every one of them I forget is a tender mercy.”

Some names relating to Robin’s case have been changed

• Follow the Long Read on Twitter at @gdnlongread, or sign up to the long read weekly email here.