Detectives investigating the baffling abduction of California mother Sherri Papini flew almost 4000km to the city of Detroit, Michigan in the days after she went missing.

Expense reports filed by the Shasta County Sheriff's Office show that two detectives travelled to the Detroit suburbs of New Hudson, Northville, Plymouth and Canton between November 9 and November 11 last year at the height of Ms Papini's disappearance.

They were obtained by The Sacramento Bee under the California Public Records Act after the newspaper lodged the equivalent of Freedom of Information requests for all reports relating to the case to counter a media blackout imposed by Sheriff Tom Bosenko.

Papini was kidnapped while jogging near her home in the small town of Redding in northern California on November 2.

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She was found beaten, branded and half starved on the side of a highway in Yolo County - more than 200km from where she was last seen - in the early hours of Thanksgiving Day.

The petite blonde told police she had endured repeated abuse at the hands of two Hispanic women during her 22 days in captivity.

The 34-year-old has been unable or unwilling to provide a detailed description of her attackers, making it impossible for police artists to render composite sketches and fuelling widespread speculation of a hoax.

Why did Sherri Papini investigators travel almost 4000km from Redding in northern California to Detroit, Michigan at the height of her disappearance?
Why did Sherri Papini investigators travel almost 4000km from Redding in northern California to Detroit, Michigan at the height of her disappearance?

Papini claimed she never got a good look at the women because they kept their faces covered - or covered hers - for most of the time.

However, some have theorised that she is withholding crucial information relating to her abductors' identity out of fear that they will come back for her or her young children.

One of the strongest theories is that Papini - who looks young for her age - was taken by sex traffickers and supporters point to her branding as evidence of this.

Professionals who have worked with former sex slaves say branding is a common practice in the industry. The FBI lists branding as a "physical indicator" of sex trafficking, used by perpetrators to declare "ownership" of the victim.

Sheriff Bosenko refused to discuss what lead detectives to Detroit or what clues, if any, they had found there.

But a statement issued by his office two weeks after Papini vanished said detectives were searching for information that would indicate whether her disappearance was "voluntary or involuntary".

To that effect, they were combing through bank records, social media accounts, mobile phone records and emails. They were also interviewing family, friends and individuals with whom she had had past relationships.

"This type of follow-up has taken detectives out of state in the hopes Sherri could be found," the Sheriff's statement said.

News.com.au can reveal that the Michigan trip may hold significance for another reason.

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A "missing" sign for Sherri Papini. Photo / AP

On November 18 - just one week after the search for Sherri Papini led two Shasta County detectives to Detroit - police in that city busted a major sex trafficking ring.

A raid on a home owned by Richard Knider in the suburb of South Electric resulted in a horrifying discovery; Knider had kept four women, two of them underage, imprisoned inside the property.

He allegedly plied them with drugs, forcibly injecting them with heroin until they were hooked and then selling them to clients for sex.

Two teenage girls Knider allegedly trafficked managed to escape from the house just hours before it was raided. They remain in hiding despite numerous national appeals for them to come forward so they can receive assistance.

Sheriff Bosenko has declined to say what investigators think of the sex trafficking theory and the timing of the Detroit trip may be nothing more than coincidence.

Neither authorities or family members have provided any insight into a possible motive behind Papini's kidnap or why her captors risked arrest by releasing her instead of killing her when they had the chance.

The case became an international media sensation, spawning dozens of conspiracy theories and dividing the public into two main camps - the believers and the nonbelievers.

Scepticism intensified after Cameron Gamble, a Redding-based "kidnap and ransom expert" offered Papini's captors a six figure cash sum in exchange for her safe return after he was retained by an anonymous donor.

The tactic, known as a "reverse ransom", failed in the sense that the kidnappers never took the money but Gamble's ransom video went viral and generated a blaze of publicity. Papini was found less than 48 hours later. Authorities said there was no evidence of a direct link between her release and Gamble's video, the ex-soldier-turned-hostage negotiator claimed credit for saving her.

Meanwhile Sheriff Bosenko's news blackout continues.

On November 30, he told a press conference that no new information would be released while the case was being investigated.

Asked last week whether he expected a resolution anytime soon, Sheriff Bosenko said it was impossible to say.

"You never know on these cases," he told The Sacramento Bee.

"Tomorrow, there could be a major break in the case. You just don't know."