They lived hundreds of miles apart, and never met face-to-face until they appeared together in the dock of Bristol Crown Court, but Vanessa George, Angela Allen and Colin Blanchard knew more about one another's secrets than anyone else.
After meeting online between autumn 2008 and spring 2009, they exchanged thousands of messages via their phones and computers, some of which contained extreme and explicit images of child abuse, which one investigating officer described as "truly defying belief".
All three defendants told police they met through Facebook, and then went on to exchange images via texts and e-mails.
But because officers can never hear the real-time phone conversations which took place, they say it is impossible to know exactly who initiated the relationship.
"I don't think we'll ever know who started the cycle," said Det Supt Adrian Pearson of Nottinghamshire Police.
'Horrific and devilish'
"It will be one of the enduring mysteries of this investigation how three people who lived at different ends of the country, and came from different backgrounds and lifestyles, could meet online and then somehow start a conversation about child abuse in its most horrific and devilish form."
Officers might not know how their cyber-relationship started but they do believe George, Allen and Blanchard were "equal parties" in the crime.
According to Det Insp Tony Creely of Greater Manchester Police: "They were as bad as each other. Blanchard expressed love for each of the women [in the texts], and they would both reciprocate.
"They would all discuss sexual matters of a crude and gross nature - and then the abuse of children."
There is no evidence the images were sent beyond the three of them or any suggestion the pictures were taken for money.
Instead, officers believe the trio were involved in some kind of three-way contest - as if they were trying to outdo one another with increasingly sordid pictures.
"They were all willing participants," said Det Supt Pearson. "The texts and e-mails were becoming ever more outrageous, and the things they were talking about were accelerating all the time."
The investigation began in Rochdale in June when a colleague logged on to the laptop of computer salesman Colin Blanchard to investigate his business dealings while he was on a work trip abroad.
When he discovered images of babies and toddlers being sexually assaulted he immediately contacted police.
Blanchard had already been cautioned for downloading indecent images of children in 2002 and as a result had spent five years on the sex offenders register.
After examining his computer and other electronic devices, Greater Manchester Police arrested Blanchard on his return to the UK. Blanchard's e-mails and texts led officers to Vanessa George and Angela Allen.
The three had done nothing to encrypt or hide the contents of their messages. One officer said: "It didn't take us long to work out who else we were looking for."
Plymouth nursery worker George, 39, was the first of the women to be arrested. Her job as a childcare worker meant her arrest immediately provoked great anger and concern among parents.
Two days later she appeared in court, charged with serious sexual assault on babies and toddlers, as well as making and distributing indecent images of children.
Parents wept in the public gallery as the graphic charges were read out. Then some of them pelted her prison van with bags of flour.
In all, George took about 150 indecent images on her camera-phone. But despite extensive investigations, the police have not managed to identify any of the children, so about 30 families have been told that their children may have been among George's victims - but they will probably never know for sure.
"It is just the worst feeling you can ever imagine," said one parent who trusted George to look after her children.
"Feeling sick 24 hours a day. Not being able to sleep. Drinking during the day. Everyone was the same - mums and dads in tears."
But detectives said George remained "very calm and controlled" during questioning. Det Supt Michele Slevin from Devon and Cornwall Police told the BBC that "in some ways George was slightly unemotional".
Officers can find no evidence of George committing similar crimes in the past. "We've no explanation, don't understand why she would start to do this kind of thing. It's something that sits within her and only she knows," added Det Supt Slevin.
The second woman to be arrested was Angela Allen. Also 39, unemployed and single, she lived in Nottingham and was described by police as being "on the fringes of society".
Like George, she was charged with serious sexual assault and making and distributing indecent images of children.
Det Supt Adrian Pearson described her as "sinister and absolutely atrocious", adding that Allen had shown "not one tear of remorse".
He said: "None of the three suspects we're talking about are bathed in glory, but Angela Allen is truly evil.
"The tone and the fantasies she described in her texts and e-mails were perhaps the most graphic of them all. It was of a graphic nature you could not possibly describe, it was chilling."
Det Supt Pearson said that this case was "absolutely unique in many ways", not least because it involved women paedophiles.
"On the face of it, these three people appeared to be entirely normal. But all of them were absolutely focused on their own sexual gratification.
"The number of victims, the scale of their deceit, the way that they manipulated relationships and the cunning they have all used for their own ends is really, really shocking."