The story begins at the tail end of 2015 after the FBI was contacted by the Brownsburg police department. Hernandez was extorting sexual images from a child in Indiana, images that he subsequently posted on the public Brian Kil Facebook page. Investigators attempted to track down the user behind the account, only to find he used Tor to mask his IP.
Investigators were already aware of Kil, however, in connection with another of his victims, a minor living in Michigan. Similarly, Hernandez used threats of violence to obtain images of the girl that were subsequently classified as child pornography. But in that case, the victim was required to use a Dropbox account and on June 9th of this year, a judge allowed the bureau to use the account against Hernandez.
Officials inserted some code into a (non-pornographic) video that was sent from the Michigan victim's computer to Hernandez. When the video was viewed, the code inside contacted an FBI server with its real IP address. Agents subsequently wiretapped Hernandez's computer and installed a camera to watch the property where he lived with his girlfriend.
In this case, the FBI was fortunate to take advantage of Hernandez's moderate lack of computer literacy, and we must be grateful for that. But it also serves as a warning to us all that we need to be vigilant against such acts.