German police said Monday they have broken up what they believe was one of the largest child pornography websites in the world, with a membership of 400,000 on the dark web.

The “Boystown” platform was uncovered during a months-long investigation that officials said led to the arrests of four men in mid-April.

Investigators with Germany’s federal police BKA believe the site had been in operation since mid-2019 and provided space for users to share images of “the most serious sexual abuse of young children.”

Authorities said the site included channels where users shared tips to access the platform and how to avoid attracting the attention of law enforcement.

Police said four men have been arrested so far in connection with the site. Three were arrested in Germany and another, who was a site administrator, was detained in Paraguay. Officials said suspects ranged in age from 40 to 64.

One of those arrested in Germany, a 64-year-old man, was one of the site’s most active members and made more than 3,500 posts, investigators said.

Police also raided several properties connected to the investigation.

Investigators from Australia, Canada, Netherlands, Sweden and the United States were involved in the operation, as was Europol.

Hesse State justice minister Eva Kuhne-Hormann called the crackdown a “fantastic investigative success story in the fight against the sexualized violence of children and young people,” according to The Guardian.