Veteran BBC investigative reporter Tom Mangold got an email out of the blue one day from a woman in Mayfield, Kentucky, asking him for help to find the murderers of a teenage girl. Intrigued, he flew out to meet her soon afterwards, and stumbled into an extraordinary story.

As soon as I landed in Paducah, Kentucky, I went straight to the local wine shop and bought a case of Marlborough Sauvignon Blanc. Mayfield, 30 miles from Paducah, is a dry town. One needs to get one's priorities right.

The Mayfield Super 8, the best (and only) accommodation in town is run by a lovely Indian man originally from Wembley, north London. I was greeted like a prince, went to my room, and waited for the phone to ring.

She called just as I had finished unpacking, the voice dark, firm and rather appealing. I asked her to come up, and the moment we met I knew we were going to be fine. Susan Galbreath has a pretty face with smile wrinkles, a head of highlighted blonde and shrewd eyes that miss nothing. She was carrying a huge file of papers and after brief pleasantries, we sat in the work area of the modest room, and started talking.

Teamwork timeline 29 July 2000: Jessica Currin murdered that Saturday night

Jessica Currin murdered that Saturday night April 2004: Susan Galbreath makes contact with Tom Mangold

Susan Galbreath makes contact with Tom Mangold May 2004: Mangold flies to Mayfield

Mangold flies to Mayfield March 2007: Quincy Cross charged with murder

Quincy Cross charged with murder March 2008: Cross's trial starts

Cross's trial starts April 2008: Cross convicted

She was terrified I would think my trip had been a waste. I reassured her it had been my call from the outset. There was no ceremony, we started work almost immediately. I unscrewed the Sauvignon, we filled two plastic toothbrush cups, and she began to brief me. No time to indulge in jet lag. Susan was in a hurry.

It was clear from the very start that Susan, for all her enthusiasm, was failing to distinguish fact from gossip, rumour or supposition. A life spent in journalism had taught me one thing if nothing else - I knew from the start what needed doing first.

I let Susan talk me through every rumour, assumption and piece of half-digested tittle-tattle on how 18-year-old Jessica Currin had met her death, each one more unlikely or unprovable than the last.

At the end of the 10th uncheckable theory of how Jessica had met her death, Susan beamed at me and asked: "What do you think?"

I treated her as brusquely as I had been treated by my first Fleet Street news editor, launching into a lecture on the nature of factual - "that's factual, Susan, hard, double-checked factual" - reporting.

It was the moment we might have parted. I saw a look of hurt and disappointment cross her face, but she didn't stop listening.

Image caption Jessica Currin left a young son

Then I went from theory to practice.

"This is the most delicious wine," I told her as we sipped our way through the Sauvignon. "You're right," she said. "It's a great wine."

"How do you know?" I challenged her rudely.

"Er… well you just told me," she answered.

"How do you know I know what I'm talking about?"

"Well, you seem to like your wine, and I assume…"

I stopped her on that one word, "assume". Then, a little more gently this time, I pointed out that assumptions were for academics and warned her never to assume anything while we worked on the case. Everything had to be checked, double-checked, tested and re-tested.

Only then did I explain why our white wine really was great. I told her to read the label; that Sauvignon Blanc is a good wine especially when it comes from New Zealand and even more so when it comes from the Marlborough region. I told her to smell the cork, ensure the label wasn't fake, sip the wine and roll it round the mouth, wait for the tingle, the slightly acidic blush and the hint of gooseberries followed by the confirmation of taste glands seemingly buried at the very base of the tongue. Then, and only then, would she be able to state as a fact that this was a great wine.

Susan got it in one. From that moment on, I never needed to ask her to give me proof of something she stated unequivocally.

Slowly, we began to form an investigative team.

She left all her paperwork. Letters, phone records - the bits and pieces that form a good kick-off to any investigation - and she went home. I fell asleep reading.

The key question - was our chemistry such that we could work together, trust each other and tolerate each other's company - had been answered.

Image caption Tom Mangold and Susan Galbreath in Mayfield

We needed to establish as best we could why it was that the Mayfield Police Department (MPD) had made a hash of the case - cock-up or conspiracy? What was the true motive for the murder? And who were the perpetrators - the perps as everyone calls them in the US?

The next day Susan took me to meet ex-detective Tim Fortner, the man who had been hand-picked to investigate the murder. He was a humble uniformed cop, a former deputy-jailer with no background in detective work. This was his first case.

I didn't have a clue what to do next - I have no idea how to organise a crime scene or look for forensic evidence Investigating officer Tim Fortner

He had been sent to the playing field of Mayfield Middle School where the body had been left in late July 2000. "I thought it was a discarded mannequin at first," he told us. "Then I saw the flies."

He retched as he got closer. In front of him was the fast-decomposing body of a black teenage girl. Her clothes and skin were badly burned, her underwear ripped off, her face a dreadful death mask of pain, eyes bulging, her tongue forced out of her mouth, and the charred remains of a belt tied tightly round her neck. A plastic bottle smelling of petrol was at her feet.

Fortner just stood there in his patrolman's uniform. "I didn't have a clue what to do next. I have no idea how to organise a crime scene or look for forensic evidence. Frankly I was scared stiff," he said.

Susan had been having coffee with a friend at a nearby cafe. Someone came in and shouted: "Have you seen the body at the school?" Susan left to find out what was happening. It was an early August afternoon. She remembers walking past the crime scene tape and into the school field, passing through a tunnel of trees to a clearing where she looked up and saw the body. At that moment she had some kind of awakening, still unexplained. But from then on, she dedicated herself to finding the murderer.

Susan's life had not been charmed. She had little education and no trade. She had left her native Chicago and married, then divorced, a violent alcoholic in Mayfield. Her life had become purposeless and she was only in her early 40s. She was a restless and intelligent woman with no hinterland to speak of and no horizon, suspended in the vacuum of an unhappy and unfulfilled life. What nobody realised was that there was an Erin Brockovich hiding inside her.

The Currin case Body of Jessica Currin, 18, left in playing field of Mayfield Middle School in late July 2000

She had been sexually assaulted, a belt tied tightly around her neck and her body set alight

Mayfield Police assigned the case to former deputy-jailer Tim Fortner, who had no experience of murder investigation

Jeremy Adams, father of Currin's son, was incorrectly accused of her murder

After seeing Currin's dead body, Susan Galbreath became determined to solve the case

The novice detective, Tim Fortner, had been given the case by his boss, Ronnie Lear, then assistant police chief of Mayfield. Lear thought Fortner was "not the brightest bulb in the package", as he later told another investigator, but said no-one else was available. Others have disputed this. Two other more experienced Mayfield detectives say they could just as well have taken on the case. Lear's role in this affair remains opaque. He later resigned from the MPD for other reasons and no longer talks to the press.

As the months rolled by, the Mayfield police made the Keystone Cops look like Sherlock Holmes. They arrested and charged an innocent man with the murder. As the trial collapsed, they then seemed to stop trying to solve the case. Fortner, disgraced, quit the force and became a security guard.

By contrast, Susan's energy redoubled. As she failed to make progress, she appealed to anyone and everyone from Oprah Winfrey to Julia Roberts to come to Mayfield to help her out. Having seen some of my Panorama investigations on local cable television, she sent me her cold email, which is how, nearly four years after the murder, I became involved.

The hapless Fortner agreed that his appointment by Lear defied logic. He also admitted that:

vital evidence had gone missing between the crime scene and police HQ

rape evidence had been shoved into an evidence box where it intermingled with rape evidence from another case

Jessica's clothes, probably containing vital DNA and forensic clues, had, unbelievably, been burned.

Susan and I now started to organise. She procured an old rusty bike for me and brought it to the motel in a borrowed pick-up. I needed to understand Mayfield, to talk to real people, and that can't be achieved by driving a hire car around town.

I also needed to speak to Jeremy Adams, a young Mayfield layabout who had been falsely accused of Jessica Currin's murder. Susan drove me to the town jail where Adams, the father of Currin's young son, had been incarcerated for a petty drugs offence. I produced an out-of-date BBC ID card to get in. Adams, who did not look like a killer, sat uncomfortably behind bars but gave me some very helpful names.

On my exit from the jail, the warden shouted at me that my card was out of date. I apologised. He replied that if he ever saw me again in Mayfield he'd have me arrested and thrown into jail with Adams. There were parts of Mayfield I grew not to like.

Image caption Donna Adams: A Kentucky accent fit to melt titanium

Susan recommended we visit Adams' mother, Donna, in her trailer on the other side of town. "Donna knows everyone, is pretty paranoid, but what she doesn't know she can find out. She's a great lady but she knows the dark side too," Susan advised.

As we walked slowly through the trailer park, I could feel eyes boring into the back of my head. This was not a place where strangers who look like retired cops are always welcome. Some of the caravan inhabitants opened their doors and began to follow us, in what was a not unmenacing manner. The men had tattoos, the women wore leather minis. Everyone seemed to have a pitbull on a short chain. As we approached Donna's trailer followed by this small, grim-faced group, Donna stepped out and stood at the top of her trailer steps. I clutched Susan's arm. I have neither great pride nor courage.

Donna is a lady beyond benign parody. She is more than 6ft tall, built like a telephone kiosk with bottle-bleached hair on black roots - an imposing and slightly terrifying lady in her mid-50s with a Kentucky accent fit to melt titanium.

She saw and recognised Susan, knew who I was, and turning to the crowd behind us, shouted two words - the second of which was "off". Even the pitbulls turned tail and slunk back to their trailers.

Image caption Annual rallies in Mayfield called for the murder to be solved

Inside her crisp, neat home, Donna opened a single malt and drank from the bottle before offering me a slug. She had a lot to say. Trouble was, I couldn't get used to her accent and she couldn't to mine. "One people separated only by a common language," as I believe Churchill once said of the Brits and the Americans. Susan acted as interpreter until Donna and I slowly began to acclimatise and communicate.

Livid with the way her son had been treated by the MPD, Donna became a gold-mine of tips, hints, innuendoes, rumours, gossip and occasional facts. Some names were now recurring, and slowly but clearly we began to discern a picture of the other side of Mayfield - its street life, its druggies, its dealers and its street girls.

One man in particular, Quincy Cross - a drug dealer who lived just across the border in Tennessee - was mentioned in nearly every conversation.

After the Donna visit, Susan and I toured Mayfield and met youngsters who had been to a large, noisy alcohol-and-drug-fuelled party on the night of the murder. Slowly a hazy scenario began to take shape. Cross had been there, so had a close friend of Jessica's, a teenager called Vinisha Stubblefield.

By complete coincidence, one evening as we left a diner in Susan's car, we passed Vinisha walking along the main street. Susan slammed on the brakes, leapt out of the car and propelled Vinisha back into it. We drove her to a coffee shop and invited her to join us for a chat.

The three of us sat down, and Susan, anxious to show me how much she had learned, began to quiz Vinisha aggressively, effectively accusing her of knowing who the killers were and possibly being one of them herself. Vinisha, a young and streetwise loner, was a proficient liar at the best of times. She simply denied knowing anything, agreeing only that she had walked with Jessica as she was going home on the night of the murder, and had then left her.

As we drove back to the motel, Susan and I had our first and last row. I pointed out that this was not the way to deal with putative suspects or witnesses, that no-one ever confesses easily or quickly. And that subtlety, tact and rat-like cunning were the only tools we had to quarry the truth.

Susan was already way ahead of the police - and unstoppable

I urged her to emulate the wonderful Colombo, the once famous TV detective in a grubby raincoat, who appeared to be a bumbling incompetent and gave the impression of being too daft to catch a parking offender. Yet, in the last reel, just as he was leaving the suspect's room, he would turn and say: "Just one more thing, sir..." and proceed to conduct the killer interview that invariably led to the man's arrest.

Susan took the point well, and indeed made effective use of tact and cunning in the months to come.

She established beyond doubt that on the Saturday night of the murder, Quincy Cross, the drug dealer, had attended the big party where alcohol, marijuana and cocaine had been in abundant supply. Susan procured - don't ask me how - a statement that the host of the party had made to police about Cross's bizarre behaviour at the party: "He kept saying he wanted to find women. He repeated this over and over - he was wired and never stopped talking."

The priapic Cross was wearing a black braided belt that evening. He vanished ("to find women") in the host's car. Susan, now on a roll, also managed to locate an instructive report from Mayfield's deputy sheriff, Mike Perkins. He had written a formal statement saying that at 07:50 the morning after the party (and the morning after Jessica's murder) he had spotted a car with Cross at the wheel. It had broken down. Perkins questioned Cross, and noticed that his clothes stank of gasoline - and that he was not wearing a belt.

Perkins allowed Cross to return to the party, which was still in full swing. A picture of that night was now beginning to come into focus. Incredibly, there's no evidence the report written by the deputy sheriff was given to the MPD. The sheriff and his staff are responsible for law and order in the county. The police are responsible for law and order in the town. They weren't working together.

We went back to see Fortner, the original police investigator, who now told us a very odd tale.

In the early days of the investigation he had actually made some real progress, despite his lack of experience. He had established that a young girl called Victoria Caldwell, a possible witness to the crime, was now terrified of being murdered herself. Fortner had immediately and sensibly placed her under police protection, putting her in a female MPD colleague's home. Then suddenly one night, without warning, she had vanished - from the home and from Mayfield. No-one at MPD seemed terribly concerned that a key witness and possible suspect had scarpered.

We were tearing around asking questions, behaving like a cross between a detective team and a couple of weird private eyes

As my short stay in Mayfield drew to a close, I observed Susan's work with growing admiration. I told her that my first Fleet Street boss had once said to me: "Tom, any clown can write a story. I don't need reporters, I need operators." I told Susan she was turning into an operator. She glowed.

More than that, I saw a Susan showing increased pride and purpose. She was already way ahead of the police - and unstoppable.

We made a strange pair in Mayfield. I'm quite tall with grey hair, I had a slight limp from a knee operation, and my accent is incomprehensible to many Kentuckians. Susan is short, dyed blonde and feisty. There we were, tearing around asking questions, behaving like a cross between a detective team and a couple of weird private eyes, returning most evenings to the lobby of the motel to drink Sauvignon Blanc from plastic cups and discuss the day's work.

Mayfield is a sad place of some 10,000 people, racked with unemployment since General Tyre left town. A place without a soul, stocked with dreadful fried-food cafes (they even fry fresh green beans in batter).

Most of the local shops have died, dwarfed by a giant Walmart, whose multi-acre car park has become a kind of al fresco social centre.

Yet we were invariably treated with kindness and respect. My humble motel often accommodated Hells Angels and their black leather-clad girlfriends, all of whom had interesting stories to tell or were interested in me as a stranger.

After two weeks of hard research, head-banging, document-collecting and interviewing, Susan and I had enough evidence to indicate that Quincy Cross and others had probably murdered Jessica in a sex-and-drug-fuelled attack. With the MPD still in disgrace, we took our evidence to Post One of the Kentucky State Police (KSP) just outside Mayfield.

A bemused trooper listened to our story, took our paperwork and waved goodbye. Susan and I both felt some foreboding that this too might not be the best path to arrests, but we had no other option.

Susan saw Quincy Cross twice outside her home staring at her - she decided she would confront him

We parted with big hugs and some tears and I promised to keep "running" Susan from west London. She was determined to plough on, work with the KSP and ensure that Cross was brought to trial.

Back home, most nights when I was not making documentaries for the BBC, I went to my study and called Susan and we reviewed her day's work, and I gave her what input I could. We worked as well as a team separated by 3,500 miles as we had when we worked together.

The KSP investigation seemed to be going nowhere.

Then one evening Susan dropped a bombshell. Quincy Cross had been stalking her. She'd seen him twice outside her home staring at her. Instead of being terrified, Susan had decided she would confront him.

I went wild - my wife recalls me shouting at Susan on the phone for more than 10 minutes. I told her to do no such thing and to report the stalking to the KSP. She refused at first, then compromised - she would tell the KSP but she would also meet Cross on a pretext. Susan doesn't do frightened.

I was obliged to work with her on a small deceit. She would tell a cousin of Cross, who she happened to know through a friend, that I was writing a book about the Currin case, and this was Cross's chance to clear his name. Susan would tell him she was my book researcher. It was corny but it worked. I begged her to inform the KSP of the venture and she agreed.

The KSP decided to fit her with a wire and wait round the corner in an unmarked car. She would use the code "I wish my brother were here" as an emergency signal for help.

So, on 25 February 2005, she arrived at the home of Cross's cousin in Mayfield. Cross was waiting alone.

Susan's own account of that meeting is instructive.

"I knock on his door, he opens it. The moment I see him and size him up, I know I can take him in a fight and I lose any fear of him.

Image caption Quincy Cross with Kentucky police

"As we sit down and start to talk, I remember your advice so clearly. You kept telling me, 'No-one ever confesses, the best you can do is to try and con them into saying something they should not have revealed.'

"He was really rather friendly... I'd expected a thug but he wasn't. Whenever I brought up the subject of Jessica's body he would say, 'I don't want to hear that shit' and he'd get up and pace round the room. At one stage he said to me: 'I'm telling you everything and you're telling me nothing. Like, did they find any DNA on that [gasoline] bottle?'

"As far as I could recall, no-one had ever mentioned the gasoline bottle found next to Jessica's body.

"We talked some more, then suddenly he told me he knew exactly what kind of belt had been round Jessica's throat. 'It was just like this,' he said, and pointed to a black braided belt he had round his jeans. And then I knew I had him, because no-one had ever revealed publicly that it was a black braided belt."

The body had been too horribly burned for anyone who saw it afterwards on the playing field to discern the type of belt - this had only been established by investigators. But Susan had seen the police files.

The meeting was a triumph for Susan, an act of real courage and she acquired important information. But the KSP deemed it to be still insufficient to move against Cross. This was a serious blow, and even Susan began to lose heart.

It was now more than five years since Jessica had been murdered. Most of Mayfield seemed aware that Quincy Cross appeared to have been involved. Most leads pointed directly to him, yet he remained a free man.

The Mayfield Police Department was off the case, the Kentucky State Police investigation was mired. Susan kept throwing stones into the pond but the ripples were having little effect. We spent hours on the phone discussing tactics and fresh clues, but we both knew the murder of Jessica Currin was fast becoming a cold case.

But then, at our lowest point, when we were least expecting it, came one of two major breaks.

Image caption Jessica's parents Joe and Jean Currin

In the autumn of 2006, Joe Currin, Jessica's dignified father and the former fire-chief of Mayfield, managed to convince the state's attorney general to assign the case to the Kentucky Bureau of Investigation (KBI), the state's version of the FBI. At last, enter an experienced detective by the name of Bob O'Neil.

When he and a partner started researching the case, he found the local MPD so hostile, he demanded and received bodyguard protection from federal marshals while in town.

That same autumn, Susan did something very clever. She created a special website called Justice for Jessica, ostensibly to honour the memory of the murdered teenager, but in reality to create a forum which might just coax a new witness to come forward.

I will help the police as much as I can but I really don't know who to trust, I am afraid someone might kill me if I testify to things about this, can you help me? Victoria Caldwell's message to Susan Galbreath

The second major break came six months later, in February 2007, when Susan's plan finally and spectacularly bore fruit.

The website received a hesitant communication from a young lady in California. She was frightened, deeply cautious, but needed to talk to someone about events that had been pressing on her conscience for many years. Her name? Victoria Caldwell - the young woman who had mysteriously vanished while supposedly under police protection.

Susan managed to secure Victoria's telephone number. She called her, and under gentle questioning, Caldwell admitted she knew what had happened to Jessica.

Seven years after seeing the grotesque body in the Mayfield school field, Susan now had the key to the solution.

She called Detective Bob O'Neil of the KBI. Then she called me.

The rest happened quickly. The KBI sent O'Neil to California where he collected Victoria. She made a full confession under caution.

This is the story Victoria told.

Jessica had been walking home from a quiet evening out when Cross had picked her up in a car. Caldwell herself was a passenger, in the back seat, and observed everything that followed.

Cross sexually assaulted Jessica and when she resisted, he stopped the car, reached under the driver's seat and pulled out a small baseball bat. With this, in a few powerful blows, he knocked her unconscious.

He then dragged her into a friend's house where he and others, including Caldwell and Vinisha Stubblefield, had already been drinking alcohol and getting high on cocaine. There, in the course of a second frenzied sexual assault, he throttled Jessica with his belt and beat her to death with a spanner.

Image caption Victoria Caldwell points out where they left Jessica's body

Caldwell and Stubblefield had wanted to leave, but Cross had stopped them - he wanted everyone there to play a part in the assault, to ensure that they all stayed quiet afterwards. The decomposing body was kept in a garden shed for a day and a half, then taken to the school playing field.

Nearly eight years later, Cross got life without parole. Victoria, in return for giving evidence against Cross, received a short sentence for helping dispose of the body, as did Vinisha Stubblefield. The others involved received various long prison sentences.

The sentences Image caption Quincy Cross, right, was found guilty of murder; Jeff Burton was sentenced to 15 years for manslaughter, abuse of a corpse and tampering with evidence Quincy Cross (pictured right): guilty of murder, sentenced to life in prison

guilty of murder, sentenced to life in prison Tamara Caldwell: 10 years for manslaughter and abuse of a corpse

10 years for manslaughter and abuse of a corpse Jeffery Burton (left): 15 years for manslaughter, abuse of a corpse and tampering with evidence

15 years for manslaughter, abuse of a corpse and tampering with evidence Victoria Caldwell and Vinisha Stubblefield: took deals for less than 10 years behind bars in exchange for testifying against Burton and Caldwell

As the mystery unravelled, Susan and I spent our evenings on the phone sipping Sauvignon Blanc and quietly patting each other on the back. It had been Susan's dedication, persistence and operating skills that had broken the case where all the detectives in Mayfield and the state police had failed, through neglect and inefficiency.

In March 2007, Susan was summoned to Francfort, Kentucky, to receive the first-ever Kentucky Citizenship Award at a public ceremony.

I would like to say the story has a happy ending but that's not the case. I returned to Mayfield in 2011 to work once more with Susan and try and get answers to some outstanding questions, including why the MPD investigation had been so grotesquely ineffective. What we discovered poses troubling questions about the Mayfield police but there is still more to uncover.

Meanwhile Mayfield has not quite showered Susan with gratitude. When she applied for the town reward for providing evidence leading to the arrest and conviction of Jessica's killers, she discovered the money had gone missing - apparently it had already been spent by the town hall.

Few local people thanked her for her seven-year campaign. There were some hostile emails and blogs. Mayfield is a curious place.

Image caption Tom Mangold with Susan Galbreath and family

I felt there was a danger that after the resolution of the Currin case, Susan might fall victim to depression. Her marriage had ended. She was again without purpose.

I used whatever influence I have on this remarkable lady and urged her to get out of Mayfield and start a new life somewhere, anywhere, else.

Last summer she finally moved to Paducah, the much larger town 30 miles away. She's looking for work, and she's taken the first step in a long journey of rediscovery. Knowing my good friend as well as I do, she will succeed.

We still talk at length, not every day, but most weekends. And she gives me advice on my life now. I work too hard, she tells me. I should relax more, enjoy life, take more holidays. We gossip.

What a wonderful partnership.

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