Image copyright Iain Maitland

After a long career in television, Bill Proctor could have spent his retirement out of the spotlight. Instead, he has been trying to track a killer.

The clock was running down on Bill Proctor's 40-year career in broadcast news.

His colleagues at WXYZ-TV, an ABC-affiliate television station in Detroit, Michigan, were already compiling his "greatest hits" reel, with the foregone conclusion there were no more great hits to come.

Then Proctor's phone rang. A strange woman's voice told him she had information about a murder.

In the mid-1990s, "Amanda" - who asked the BBC to withhold her name - was a crack addict living on Detroit's west side. She claimed that, one night, in the middle of a smoking binge, a man she described as a casual boyfriend left her apartment to buy more drugs from a dealer in the building. When he returned, she told Proctor, his hands and jacket were covered in blood.

"I think I killed her," she recalled him saying.

Image copyright Iain Maitland Image caption The apartment building where Christina Brown was killed has since been razed

Amanda said her boyfriend forced her to gather a few things and flee to a house he shared with his brother a few blocks away. For weeks, she said, he menaced her and wouldn't let her out of his sight. When it became clear the law wasn't looking for him, she said that he started to relax, and she was able to slip away.

The police, she would find out much later, had arrested a different man.

Proctor listened intently. It was a crazy story told almost two decades later by a woman who admitted she was high at the time. But she had been trying to get someone to listen to her for years, she said, and had even gone in person to the police in 2012 to file an official report. But nothing seemed to have come of it.

She gave Proctor the boyfriend's full name, then told him something that made his stomach turn.

"You were the investigative reporter on the case," she said.

Media playback is unsupported on your device Media caption An early news report on the death of Christina Brown (courtesy WXYZ)

In the last years of his career at WXYZ, Proctor had gone from a beat reporter dutifully chronicling the city's daily chaos to a well-regarded investigative reporter. Past stories by Proctor brought down a judge and a Detroit police chief who was spending his time shopping around for a reality television show.

In the process, he had become deeply sceptical of the criminal justice system, to the point where he'd founded a non-profit to help victims of wrongful conviction.

But now, if Amanda was to be believed, it was Proctor who had got the story horribly wrong.

After he hung up, Proctor - a tall, smooth-pated 67-year-old who speaks at all times with an anchorman's baritone cadence - headed to the station's archives. He pulled out a dusty tape from January 1996. A mother's tear streaked face appeared onscreen.

"She never got in fights in school, she always came home, you know, after school, she never stayed out late until, until this," she stammers.

Then Proctor heard his own voice as the screen flashed to an image of the young victim. The picture quality was so poor that her face was just a blur.

"Christina Brown had apparently grown up very fast," the decades-younger Proctor boomed. "Her mother says she never suspected that her 12-year-old was out selling drugs, only that her running away for weeks at a time meant trouble."

The tape jogged Proctor's memory - he had indeed been the lead reporter on the story, as he had scores of times when Detroit was the murder capital of the country and the victims were often astonishingly young. In those days it was almost rote - he heard about a body over the police scanner, tried to score an exclusive with the next of kin, got the details from a Detroit homicide inspector and then delivered the story in terms that were at once dramatic and detached. Then he moved on.

"In the 1990s, when the murder rate was skyrocketing and so many people that ended up dead were in the drug business, this was just another horrific element of what was going on around us all the time," he says of the Brown murder. "I don't think I looked at this as being special."

Image caption LaMarr Monson

The final instalment of the Brown story showed a few seconds of the arraignment of LaMarr Monson, the 23-year-old drug dealer who immediately incriminated himself at the scene by giving police a fake name. He was eventually convicted and sentenced to 30 to 50 years in prison.

On the tape, Proctor characterised Monson as a man who "lured" Brown away from her family, and may have had a sexual relationship with her, "stealing her innocence and eventually her life".

"There was a confession, so police investigators have a pretty good idea why this murderous assault took place," Proctor says at the close. "At police headquarters downtown, Bill Proctor, Channel 7 Action News, reporting."

Something in the old footage made him sit up and take notice - it was the homicide inspector he interviewed, a woman named Joan Ghougoian. She appears briefly, telling the younger Proctor: "The cause of death was the result of multiple stab wounds."

Anyone familiar with the troubled history of the Detroit police department knows Ghougoian's name - a history that includes 11 years of federal oversight after the US justice department found Detroit police were shooting and killing more civilians than any other agency in the country and regularly violating the constitutional rights of suspects.

Less than a year after she granted the interview for Proctor's piece, Ghougoian herself was the subject of headlines, removed from her powerful position as one of the first female heads of a major city homicide unit in the country.

Her superiors found that she coerced illegal confessions from innocent men, and then leaned on her subordinates to cover it up. She was implicated in a half a dozen potential coerced confessions.

Ghougoian denied the charges and retired from the police force in 1999.

As soon as he saw Ghougoian on the tape Proctor became suspicious of his own report on Monson's confession. He began hastily arranging a follow-up piece - he interviewed the witness, tracked down Monson's mother, and called Monson himself at his prison outside Detroit.

By the time he was finished, he had convinced himself that Monson was innocent, but not WXYZ station managers. They weren't interested in a follow up.

Proctor was crushed. He now saw the story as a blemish on an impeccable 33-year record in Detroit.

When his retirement day arrived, Proctor signed off, packed up his plaques and Emmys, and drove it all to his modest house in a suburb of Detroit. In his boxes was everything from the unaired story about the Monson case.

That was two years ago.

Since then, he has been in a constant investigation that led him on a cross-country search for the man he believes is the real killer. The case has proven to be a perfect segue into his so-called retirement. Though he may have traded his blazer and neckties for golf shirts and a Washington Nationals cap, Proctor works more than full-time in his second career investigating suspected cases of wrongful conviction in Michigan.

"This case is an attempted vindication," says Proctor. "I have to feel that I corrected a really sad miscarriage of justice."

Proctor was not always the kind of reporter who was quick to condemn the police and the system in which they operate. He used to be one of them.

"I knew there was a difference between right and wrong, and there was the stupidity of living in a free country and throwing it away," he says. "Things kind of were in a fairly straight line in my head at the time."

In the early 1970s, he was an officer with the Federal Protective Service, the uniformed, armed security force for federal buildings and land in Proctor's hometown of Washington DC. He took the job to help pay his way through college, and support the daughter he'd had when he was just 18.

Image copyright Iain Maitland

During the day he clashed with anti-Vietnam war activists while wearing a white helmet and carrying a baton. In the evening he sat with those students in class at the University of Maryland.

He became obsessed with becoming a television reporter after meeting Hal Walker, the first African American reporter at the DC bureau of CBS. His first job in television was as a copy boy at WMAL in Washington DC, sometimes working for weekend anchor Charlie Gibson, who went on to host Good Morning America.

"It was a time when there weren't many African Americans in the business and so you wanted to reach out to those guys," recalls Gibson. "He was quite bright. He had a very good presence that I thought would one day end up on the air."

It didn't take long - Proctor landed his first on-the-air reporter job in affluent Lynchburg, Virginia. It wasn't an easy place to be a young black reporter in 1974.

"The general manager of the station kind of gave me a tour and it was one of those tours where you were shown where you were allowed to go and where you weren't welcome," says Proctor. "I couldn't go to the country club. I literally was not allowed in the YMCA. He just took me there to show me, 'This is where the door is and you can't go through it.'"

Image copyright Iain Maitland

As he moved up the television market food chain from Kansas City to Pittsburgh and finally Detroit, Proctor evolved from a cocky young anchorman who relished the trappings of TV celebrity into a seasoned news reporter so cool under pressure he became known as "the Doctor".

His background in law enforcement was well known enough that he was once called in behind police barriers to help negotiate the release of three officers being held hostage by gunmen at a seedy motel - though it was hardly a triumphant moment.

"I was way too late. They had already killed them and stuffed their bodies in a closet," says Proctor.

In 1995, while he was still on general assignment for the station, Proctor was approached by a grizzled private investigator. He had been hired by the girlfriend of a man named Frederick Freeman. Freeman was doing a life sentence for murder. The investigator was at the end of his rope - he was sure that Freeman was innocent, but couldn't get anyone to listen.

Freeman, who now goes by the Buddhist name Temujin Kensu, remembers the day Proctor first visited him at the Macomb Regional Correctional Center.

Image copyright Michigan Department of Corrections Image caption Temujin Kensu, aka Frederick Freeman

"He said, 'Well, I've got all day,' in that voice of his," Kensu says "'I'm going to know if you're lying to me.' He sat there for at least six hours. Filled a yellow tablet front to back."

Kensu was convicted of killing the 20-year-old son of a small town mayor with a single shotgun blast, despite the fact that multiple witnesses testified he was 400 miles away at the time of the shooting. A woman who had dated both men says Kensu influenced her with mind control, leading the press to dub him "The Ninja Killer".

Proctor was enthralled. He reinvestigated the case for a year, turning the results into a five-night series on WXYZ. He managed to find a key prosecution witness - a jailhouse snitch - who recanted on camera.

"It woke me up," says Proctor of the case. "I was just another journalist walking from story to story, and the only time I was running was when there was a fire and you had to get there before the fire went out."

The story garnered national media attention and the support of many politicians and judges. In 2010, a federal judge granted Kensu a new trial, but the decision was subsequently reversed by an appeals court judge in 2012. Prosecutors say that the evidence against Kensu is compelling and their witnesses trump his.

Proctor continues to visit Kensu in prison to this day.

Wrestling with the Kensu case transformed Proctor. He founded a nonprofit called Proving Innocence, which connects pro-bono investigators with inmates who demonstrate a strong case for their innocence. He decided he needed to become a licensed private investigator himself.

The Kensu case also brought him into contact with David Moran, a former state appellate defender who was looking to start a full-time clinic for wrongful convictions. He was specifically interested in cases like Kensu's that focused on factors aside from DNA evidence - lousy forensic science, prosecutorial misconduct, coerced confessions, ineffective counsel.

According to the National Registry of Exonerations, there have been 1,718 exonerations in the US since 1989. As the number of cases that rely on DNA evidence has remained fairly constant, the number of cases reversed on non-DNA evidence has tripled since 2005. There are only a patchwork of legal clinics that take on these kinds of cases.

Proctor facilitated a series of meetings between Moran and Bridget McCormack, then the associate dean of clinical law at the University of Michigan, which led to the formation of the Michigan Innocence Clinic.

"[Bill] conceived it and we tried to run with it," recalls McCormack, who is now a justice of the Michigan Supreme Court. "When there is no DNA you really have to find evidence to prove someone is innocent. A journalist is not afraid of that."

Since its formation in 2009, Moran and his students have freed 11 wrongfully convicted men and women, and taken on 30 cases including Kensu's. The students use Proctor regularly as a pro-bono private investigator, or sometimes just as a resource.

Image caption Moran and his students at the Michigan Innocence Clinic, along with Proctor

"Whenever Bill calls, I'm going to take that call," Moran says.

Proctor is always invited to the clinic's "exoneration parties", which is where - over beers and sheet cake in honour of another newly freed client - their first conversation about LaMarr Monson took place.

Proctor told Moran about the phone call from Amanda, and gave him the name of her boyfriend - Robert (the BBC is withholding the man's full name as he has not been charged with a crime).

Moran dug through their database of cases and found that Monson had already sent in a letter asking for the clinic's help in 2009, but it had languished in a pile of other cases marked for further investigation.

Now, with a new theory of what actually happened to Christina Brown, Moran began digging through Monson's case files to look for evidence that Amanda was telling the truth.

The documents laid out a dark and convoluted story.

Image copyright Michigan Innocence Clinic Image caption Evidence photo from the scene of Christina Brown's murder

Christina Brown's body was discovered on 20 January 1996. Her death was horrific. She had been stabbed 17 times. She was strangled. Her jaw had been broken with enough force that some of her teeth were knocked out.

The blood in her stomach showed that she had lain on the floor of the freezing abandoned apartment at 2752 West Boston for hours struggling to breathe, surrounded by trash and soiled clothing.

Around noon that day, witnesses told police that "Mark" - the alias Monson used when he was selling drugs - had run down the hallway of the building, banging on doors and asking someone to call 911.

By the time paramedics arrived, Brown was unresponsive. She died not long after.

The other tenants of the building - who knew Brown as "Crystal" and thought the 5-foot-10-inch girl was 17 years old - told police that Monson and Brown were selling drugs from the apartment.

Once he was taken down to the station, Monson maintained that though he had found the body, he had nothing to do with Brown's death. He said he spent the night of her killing at his on-again-off-again girlfriend's home with their seven-year-old daughter. The girlfriend would later corroborate that story.

But about 24 hours later, Monson allegedly dictated and signed a detailed confession. The type-written document said he had actually returned to the apartment drunk in the middle of the night, and he and Brown began arguing over sexual infidelity.

Image caption The second Monson statement to Detroit police - which Proctor believes includes a false confession

"Crystal then charged at me with the knife in her hand," the statement reads. "[I] pushed her head through the window in that bathroom, breaking the glass… As I was pushing her back, the knife was bent in her hand, and it stuck her in the neck…Crystal then collapsed on the floor of the bathroom, the knife fell in the sink. I then left the apartment."

When Moran compared the testimony of the medical examiner to Monson's confession he saw his first major red flag. Monson's confession says he stabbed Brown in the neck, and left her to bleed to death.

But the medical examiner testified at Monson's trial that the stab wounds were mostly superficial. He determined the murder weapon was the bloody toilet tank lid that police found discarded on the living room floor of the apartment. The killer had fractured Brown's skull with it.

Monson's confession contained no mention of a beating with the toilet tank lid. He wrote in letters not long after his conviction that former inspector Ghougoian had promised him he could go home if he signed the statement, which he says he did not write.

"The interrogation of Monson occurred before there had been a complete forensic analysis, so the detective didn't know what facts they needed to get out of Monson in this confession," says Moran. "So the facts that they got don't match up. That's a marker for a false confession."

There was more. A man calling himself "Raymond", but identified by multiple witnesses as "Robert", was interviewed by the police the day of the murder. According to the report, he told an officer that he was staying with Amanda in the building, and that he regularly bought drugs from Brown.

Amanda told Proctor that Robert flew into a rage after he attempted to buy crack on credit, and Brown rebuffed him. In his statement to the police, Robert acknowledged he had "coped some dope" from Brown the night of the murder.

Moran also found a latent fingerprint report in the files. It positively identified a thumbprint on the bathroom mirror as Monson's. But below that was a note.

"There is still an unidentified usable print and palmprints left," it read. The prints on the bloody toilet tank lid had never been matched to anyone.

Image copyright Michigan Innocence Clinic Image caption A bent knife found at the scene yielded no fingerprints

Proctor called in every favour he could muster using the clout left over from his television career. ("Bill doesn't work for us," Moran is careful to point out. "He works in parallel.")

"I reached out to the mayor's office. I was face-to-face with the chief of police," he says. "From the mayor's office on down, they know that I'm very concerned and active in this case. I want them to reopen it and they won't."

After months of politicking and cajoling by both Moran and Proctor, a detective let slip that someone authorised the print to be sent to a lab.

But Moran says he refused to give them the report.

Months later, a lawyer from the Wayne County Prosecutor's office finally agreed to send the single sheet report to Moran.

The left thumbprint was a match for Robert, the man Amanda identified on that first phone call.

"I was ecstatic," says Proctor. "It's just another level of vindication and another reason for the absolute fury that every American should have over a case like this. The police had in their hands the very evidence they need to do more and they don't do it."

Proctor says he learned from the Kensu case that even new evidence is not enough to get authorities to earnestly re-examine their case. That's why he continues to search for Robert.

"I don't really care how he wants to explain it, but I need him to tell me - when he went back to get crack on credit from the girl, something happened," says Proctor. "I want him to tell me what he did."

In early fall, Proctor made his third trip to Johnstown, Pennsylvania, a seven-hour drive from Detroit. Using databases he has access to as a licensed private investigator, Proctor ascertained that Robert moved to the faded steel town not long after the murder with two of his brothers.

On his first trip, Proctor arrived at a house where neighbours told him Robert moved from just months earlier. On the second attempt, Proctor unintentionally ended up on the doorstep of Robert's brother. The brother called Robert and handed Proctor the phone.

"I didn't do nothing wrong," is Proctor's recollection of Robert's words.

Now on his third attempt, Proctor's tactics have become increasingly elaborate. Through informants, he found out that Robert is very ill, and Proctor has a half-baked plan to see if he can intercept him on the way into one of the tiny town's hospitals. He looks up Robert's criminal record at the local courthouse, scouring the documents for addresses and phone numbers - none are current. He drives cautiously by the brother's home, hoping to catch a glimpse of Robert. No luck.

Image caption Proctor in Johnstown

In an effort to prepare himself for the hoped-for run-in with Robert, Proctor asked a man named Walter Swift to join him on the trip. Like Kensu, Swift was an inmate whose story further convinced Proctor that the criminal justice system is broken - and that cases like these are never as simple as they seem.

In 2009, Swift made international headlines when he was freed 26 years after being wrongfully convicted for a rape and robbery. DNA evidence cleared Swift and after Proctor covered the story, the two men became friends.

Proctor thought that perhaps if Robert met Swift, a charismatic, contemplative man who had lived through the horror of a wrongful conviction, Robert could be persuaded to tell the truth about the murder.

But Swift is not in Johnstown - he is back in jail for burglarising a store, the latest in a string of arrests that could send him back in prison. Proctor had hoped to help Swift write books and give lectures. The reality of his release from prison has been very different.

"These months with Walter have been some of the most difficult of my life," says Proctor. "It makes me realise you just can't wish that things turn out the way you might have it in your own mind."

When Swift was awarded $2.5m (about £1.7m) in compensation from the bankrupted city of Detroit for his time in prison, he and his lawyers decided to appoint Proctor the sole trustee over the funds. Since his release Swift sunk into addiction. He has traded away thousands of dollars' worth of furniture for crack. He has shown up on Proctor's doorstep in the middle of the night, high and demanding money.

"There's nothing you can possibly do to keep me from using drugs especially when I'm worth $2m," says Swift now. "I'm not trying to tell you this is right, I'm trying to tell you this is reality."

Despite all that, and the objections of his wife, Proctor says he can't abandon Swift. His latest plan is to beg the judge to let him drive Swift to an in-patient treatment centre in Vermont for a year-long rehabilitation programme. Swift has agreed to go.

Image copyright Iain Maitland Image caption "Am I going to keep trying? Probably."

"I have no idea what's going to happen, but he'll be a part of my life until one of us is dead," says Proctor.

On the final day in Johnstown, the sun is beginning to lower as Proctor prepares to leave town. He has made no progress on finding Robert, and he has to start the long drive back to Detroit or he'll miss Swift's hearing.

"I'm disappointed but not really surprised."

He pauses.

"Am I going to keep trying? Probably."

When LaMarr Monson thinks about the killing that led him to lose his freedom, he has a predictable reaction.

"I'm very angry. I don't understand how you can do something like that," he says. "This was a sick individual."

Despite that, Monson, now a bald-headed, bespectacled 43-year-old man serving his sentence at a prison in Adrian, Michigan, is elated that Proctor and Moran have been able to prove many of the things he's been saying for 19 years.

"I'm finally being able to get my story out. It feels good," he says. "I didn't do this. I want that to reign above everything."

On 14 December, Proctor puts on a suit and tie, and picks up Monson's mother Delores at the tidy house in west Detroit where she raised LaMarr. Proctor drives her to a press conference two blocks from the Wayne County Prosecutor's office in downtown Detroit where a room full of reporters is waiting.

"We just want our son home. He's innocent," Delores says into the microphones. "The person responsible for this crime has been free all this time and that is unacceptable."

Thirty minutes earlier, Moran filed the motion for a new trial for LaMarr Monson, naming for the first time the man whose fingerprint was found on the murder weapon: Robert.

Image copyright Iain Maitland Image caption Bill Proctor and Delores Monson at a 14 December press conference

"Fingerprint evidence discovered in 2015 establishes that Mr Monson is innocent," the motion reads. "Mr Monson is entitled to a new trial."

Messages left for Robert were not returned. His brother denies any knowledge of what occurred and says he plans to sue anyone who implicates him in the murder.

The Detroit Police Department would not answer questions about the case, or whether they have any plans to contact Robert. The Johnstown Police Department did not return messages.

The Wayne County Prosecutor's office also declined to discuss the details of the case, but acknowledged that an assistant prosecutor is assigned to it.

"We will respond to the motion if directed to by the trial court," wrote a spokesman to BBC News.

For Proctor, the press conference was an important moment coming full circle. After Delores Monson finishes, he steps in front of the cameras for the first time since 1996 to tell Detroit that he now believes LaMarr is innocent.

"What the Monson family suffered, what LaMarr has suffered should not happen," he says. "This is an office of the prosecutor that refuses to step up and do what needs to be done in a case of actual innocence like this one."

He also delivers a message to Robert.

"Explain what happened to this child."