For two weeks in 2015, the FBI ran an unprecedented operation, where they hosted and shared child pronography from the FBI's servers.  The operation began when the FBI seized a computer network that had been hosting child pornography.  Rather than simply take down the entire network, the FBI decided to host the server, and see who came to visit the site.  The FBI applied for and received a search warrant that allowed them to  use a hacking technique know as "Network Investigative Technique", or NIT, that could infect targets who visited the site, and captured their IP addresses.

For more information on the operation click here or here to read about it.