Williamson, 42, is a senior official at the US Department of Justice and serves as a liaison to a special FBI unit for violent crimes. Her focus is cracking open cold cases – specifically murders and sexual assaults – that have remained unsolved for many years.

More than a job, it’s an obsession.

“These are the underdog cases, the people who have been left behind,” Williamson tells The Sydney Morning Herald and The Age in an interview near her office in Washington, DC.

Although she has lived in the US for more than a decade, Williamson retains an Australian accent and dry sense of humour. “There is a lot of job security here,” she says, referring to the higher rate of serial killings in the US than in Australia.

Williamson is used to working behind the scenes, but her role in the Little case has pushed her into the spotlight.

The week before our interview she was in Chicago giving a presentation to the world’s top police chiefs, and she recently featured on the US version of 60 Minutes. In August, the International Homicide Investigators Association presented her with an award for investigative excellence.

Her involvement in the Little case began in December 2017 at a law enforcement conference in Tampa, Florida. Williamson was chatting to a Texas Ranger named James Holland, an expert at interviewing sociopaths and psychopaths. The pair were approached by some local officers who asked whether they knew anything about a man named Samuel Little.

Little was in jail in California for killing three women, but the officials suspected he had also committed murders in Florida.

A few months later, Holland called Williamson and said they ought to take a look at “this Little guy”. She turned to her colleague, Christie Palazzolo, and asked if she had heard of him. She had, and suspected he may have murdered a woman in Odessa, Texas.

They had no reason to believe that Little would admit anything, or even talk to them; he had always maintained his innocence. But they decided to give it a go.

“We thought: what’s the worst that can happen?” Williamson says. “He tells us to bugger off.”

Williamson, Palazzolo and Holland travelled unannounced to the Los Angeles jail where Little was incarcerated and asked to speak to him. Holland interviewed Little while Williamson and Palazzolo observed from a separate room. (“He had some issues with women,” she notes wryly.)

The first half hour or so was unproductive but then, when asked about Odessa, Little admitted to killing a woman there. “We had this crime scene photo in front of us and he was describing it down to the colour of the victim’s shirt,” Williamson recalls. “We thought: ‘Wow, we’ve got something here!’ “

Then, over 49 straight days of questioning, Little confessed to murder after murder – more than those of Ted Bundy, Jeffrey Dahmer and the Manson Family combined. Most of the women were on society’s fringes: prostitutes, drug addicts, people he figured the police wouldn’t work hard to find.

Although Williamson’s speciality is forensics, there was next to no DNA evidence available for these cases. Her main role was combing through crime databases, searching newspaper clippings and contacting local police departments to connect Little’s confessions to specific victims.

So far the team has been able to corroborate 51 of Little’s confessed killings, and another 12 confirmations are pending. In October the FBI announced it believed all Little’s confessions are credible.

“He says he chose people who wouldn’t be missed, but he was wrong about that,” Williamson says. “The relatives are often overwhelmed to find out what happened. We notified a sister of one victim just this past weekend.”

Williamson, who earned her PhD at the University of Queensland, first came to the US in 2002 when George Washington University recruited her to help develop a vaccine for infectious diseases.

After taking a class in forensic science at the university, she decided to change her career path. This was her chance to live out her childhood dream of being Dana Scully – albeit without the paranormal elements. After a few years working in Queensland, she returned to the US to take up a role at Bode Technology. The firm operates one of the world’s top private DNA laboratories and was responsible for identifying victims of the September 11 attacks – a task that continues to this day.

While working as the company’s head of forensic case work, Williamson oversaw the DNA testing of JonBenet Ramsey’s longjohns in 2008. “I used a Queensland sampling method that other labs weren’t using over here at the time,” she says. The evidence led investigators to formally clear Ramsey’s immediate family members of her murder, which remains unsolved.

She also oversaw the DNA testing that led to the release of the West Memphis Three – a notorious group of Arkansas teenagers accused of killing three young boys – after 18 years in jail.

Asked how she copes with the macabre crimes she encounters, Williamson says: “People say there is no crying in baseball – well there is no crying at crime scenes.

“You have to compartmentalise things … For all the evil people you come across, you also meet people who are dedicating their lives to making the world better and safer.”

The public’s biggest misconception about forensic evidence, she says, is that there is “a magic computer where you put in the DNA profile and the guy’s face, social security and address pops up”.

“That doesn’t happen. It can be tedious, backbreaking work. You have to be relentless and keep plugging away.”

There have been several advances in DNA technology over Williamson’s career, but she says a recent “gamechanger” has been the growth of genetic genealogy services used to trace family trees. They allow investigators to identify criminals among the relatives of those who upload their DNA profiles.

Joseph DeAngelo, the so-called Golden State Killer, was arrested in April 2018 after police used  public genealogy databases to link him to DNA from the crime scenes.

While there are obvious privacy concerns about the practice, Williamson says it is invaluable in solving crimes and providing justice to victims’ families.

“There are 240,000 unsolved homicides in this country and that is too many,” she says. “Everyone’s story deserves an ending.”