John Eric Armstrong was living a picture-perfect life. At night, he was killing women.

John Eric Armstrong was unassuming and polite, described by co-workers as the "boy next door" with his red hair and wireframe glasses.

The bespectacled former US Navy petty officer had a young son and another child on the way with his wife Katie, who affectionately called him 'baby doll.'

His Target co-worker Jennifer accepted lifts with him to and from shifts in Detroit, Michigan, when the family moved there in 1999. She never felt afraid of the 26-year-old, and they spent their commute chatting about family, their childhoods and how much he loved his wife.

But Armstrong had a secret. After dark, he was killing women.

Watch: The 'Baby Doll' killer. Post continues below.

The body of Monica Johnson — a 31-year-old mum of four — was found abandoned and face-down in Detroit, in December 1999.

She'd been working as a sex worker and Armstrong had picked her up around 11pm, having sex with her in his Jeep Wrangler which had a number-plate that read 'Baby Doll', before strangling her.

"At some point while I was strangling her she stopped breathing, so I placed her on the sidewalk," he said later in a confession. "I then left and went home and took a shower."

"Did you intend to kill her after sex?" the detective asked him.

"No," he replied.

"How did you feel after killing her?" was the next prompt.

"I felt awful, very hurt and very sorry," he insisted.

But Monica wasn't his only victim. While Armstrong was only convicted of killing five female sex workers, it's believed he could be responsible for the deaths of up to 20 people.

The depraved crimes of the 'Baby Doll' serial killer.

After each of his kills, Armstrong would return to his family home for a nice long shower, before crawling into bed next to his wife.

As co-author of THE 'BABY DOLL' SERIAL KILLER: The John Eric Armstrong Homicides, B.R Bates told Mamamia's True Crime Conversations, "From everything I understand, his wife was not aware of how long he was even out, and I really don't think she was aware of what he was doing".

Listen: To the story of the Baby Doll serial killer. Post continues below.

Wendy Jordan, 39, was murdered in the early hours of New Year's Day 2000, after being picked up by Armstrong in his Jeep Wrangler.

He had sex with her, strangled her and threw her body into a river not too far from his home.

Armstrong was interviewed by police, after he himself alerted police to a woman's body spotted floating. They were immediately suspicious of his story.

"They [police] started processing the scene for evidence, and in all of that their eyebrows were just a little bit raised at this guy. There was something about his story that was not making sense. He claimed that he had pulled over there to vomit because he was feeling sick. What didn't make sense to them, was there was no vomit anywhere.

"They were [also] a little suspicious by something that they saw in the Jeep. Wendy had been wearing a pair of gold-coloured shoes and there were little flecks of gold on the floor on the passenger side."

They collected Armstrong's DNA, but had no choice but to release him as they waited for the results, a process that took three months at the time.

In that window, he killed three more women, dumping all of their bodies in a concentrated area near some railway tracks.

Armstrong's Jeep Wrangler with the numberplace 'Baby Doll' became a key identifying factor amongst the women who managed to escape him. Image: AAP/Carlos Osorio.

All three victims had ligature markings around the neck, and each had been posed in a sexually provocative way.

Armstrong later admitted he, "positioned them so he could always go back and have sex".

During his detailed confession, he estimated he'd killed at least fifteen times from as early as 1992. He claimed his other kills were outside of Detroit — in Seattle, Washington, Hawaii, Hong Kong, China, North Carolina, Virginia, Thailand and Singapore.

In a handwritten note at the bottom of his statement he wrote, "I might have possible [sic] done at least three more. I stopped counting at around eight. I tried the best I could to remember.

"I can try at a later date to remember how many more people I killed. I would like to apologise for all the people I have killed, and all the families that have suffered because of me. I hope I can be forgiven of my crimes. But I would like to seek help for my problems. I do remember having sex with some of the bodies after dieing [sic], such as Seattle, Hawaii and Detroit."

There were also numerous survivors, some of whom aided the police in their case against him.

Putting a killer behind bars.

Initially, his wife Katie supported him. She didn't want to believe the charges against him were true.

"That's how police described her as… just shocked, and 'this is not my husband. You got the wrong guy', and I think she honestly believed it," Bates told True Crime Conversations.

Armstrong's family home, which he shared with his pregnant wife and young son. Image: AAP/Carlos Osorio.

"But then, by the time Armstrong was convicted, she left the picture and he has not seen her. He has not met the child that his wife was pregnant with when he was arrested."

In March 2001, Armstrong was found guilty of murdering Wendy Jordan and sentenced to life imprisonment. In June, he was additionally convicted of killing Kelly Jean Hood, Robbin Brown, Rose Marie Felt, and Monica Johnson. He will never be eligible for parole.

In the writing of her book, Bates met with Armstrong in prison a few times, describing him to True Crime Conversations as, "just not what you would think… not a serial killer at all.

"That's why his arrest [and] his identification as the serial killer was so shocking to anyone who knew him here in Michigan…just so very, very shocking. His demeanour and his appearance — he strikes you as very genuine when he chats with you. He smiles, he seems very kind in a genuine way, not a put-on way or a manipulative way."

As for the motives behind his sinister kills, Bates says it's a question that largely still remains.

"Three of the survivors said that as he was strangling them, he would say something about 'hating prostitutes' or 'I hate hookers' in a very angry way," she said.

A popular theory at the time of the case, as was reported by the media, was Armstrong's high school girlfriend leaving him for a guy who bought her things which he saw as "prostituting herself".

Bates isn't convinced that's reason enough to explain a spree of violent murders years later.

"I just don't know if that makes sense…. So that is a mystery of why he so deeply hated prostitutes. It's really a mystery," she said.

Nowadays, Armstrong refuses to discuss his crimes despite his detailed confessions at the time of arrest. He's serving his time in G. Robert Cotton Correctional Facility in Jackson, Michigan, and he doesn't get any visitors other than his mum and step-dad every year or two.

Feature image: AAP/Detroit Police.