Swiss police have busted a child pornography website, based in St Gallen, operating in at least four countries.
Prosecutors in the eastern canton said four men had been arrested in Switzerland on suspicion of producing child pornography. Two had also admitted sexually abusing a young girl.
On Wednesday authorities confirmed Swiss radio reports that the victim was 12 years old and knew some of those arrested.
The four arrested are among 13 Swiss aged 20-65 who are subject to criminal investigations.
The prosecutors said the online forum was frequented predominantly by users from Germany, with some from Austria and Liechtenstein. Investigations have also begun outside Switzerland.
The arrests were the result of work by the Cybercrime Coordination Unit (Cyco) within the Swiss justice and police department.
Cyco said tips could be found in the forum on how to approach and groom young girls online and how to arrange meetings with children. It added that pornographic material had been exchanged on the site.
"The important thing is that this wasn't a hidden ring – the forum was partially public. Obviously you had to register to post, but not necessarily with your real personal data," Marc Henauer, head of Cyco, told swissinfo.
"The point is that it was really on the borders of those self-help forums. A lot of activity by paedophile groups is anonymous – not a classic ring where people meet and even exchange their children and so on."
The forum was hosted by a service provider in St Gallen but was operated by Germans, according to the coordination unit.
Henauer said he didn't know why St Gallen was targeted – "we're certainly not more liberal than other countries".
The investigation has led to the identification of 600 people in Germany, 40 from Austria and four from Liechtenstein.
The Swiss authorities have handed over their information on the users and operators to their counterparts in the relevant countries.
St Gallen investigating magistrate Ursula Brasey said the St Gallen internet service provider who hosted the site had been very cooperative and was not among the suspects.
"The investigation has been under way for several months," said Brasey, who added that the examination of confiscated materials had been complicated and time-consuming.
According to Henauer, the hardest part of such online work is separating the active, dangerous paedophiles from those who might just talk about it.
"Often there is a lot of rationalising by paedophiles that what they are doing isn't bad or at least not as bad as everyone says. That's also what these forums are used for: to make each other feel right," he said.
"It is in these shades of grey that the hard work starts. Obviously you want to have the cases where there are very clear indications that people are sharing this kind of material or are probably involved in child abuse afterwards. Filtering the cases is not easy."
swissinfo, Thomas Stephens
It is estimated that the proceeds of child pornography and child prostitution worldwide amount to more than $20 billion (SFr25 billion) a year.
According to Unicef, there are 14 million child pornography websites on the internet.
Cybercrime Coordination Unit (Cyco)
Cyco is the central office where people can report suspicious matter they find online. After an initial examination of the report and safeguarding of the data, the report is forwarded to the respective national or foreign law enforcement.
The coordination unit also searches for criminal subject matter on the internet and is responsible for in-depth analysis of cybercrime.
Under Swiss law the downloading and sale of child pornography is illegal. The sale of images is punishable by up to three years in prison. Downloading images is punishable by a prison sentence of up to one year.
Judges have the authority to add an unlimited fine to any prison sentence, with child abuse punishable by up to five years in prison.