A Freedom of Information Act (FOIA) request to the Department of Justice reveals that at least in one instance, an FBI inquiry into illegal child pornography (also known as child abuse imagery) on a Tor-protected site was apparently halted in its tracks due to the use of the anonymizing tool.

Tor, of course, masks traffic online and is used by seedy characters. But the tactic is also used by law enforcement, intelligence agents, activists, and private individuals who want to keep their online trail as hidden as possible.

"Because everyone (all Internet traffic) connected to the Tor network is anonymous, there is not currently a way to trace the origin of the website. As such, no other investigative leads exist,” a 2011 FBI Complaint/Assessment Form from the Detroit field office states.

This of course, is despite the fact that there have been several instances where law enforcement agencies have managed to crack Tor-enabled online communities engaging in illegal activity. Most recently, in April 2012, the “Farmer’s Market” illegal drug sales site was shut down. In late August 2011, Dutch authorities announced that they had cracked another child porn ring that also used Tor to hide its origins.

“Saying that you have no leads is ridiculous,” Karen Reilly, the development director at the Tor Project (the organization behind the software), told Ars on Tuesday. The FBI did not respond immediately to requests for comment about this case.

Reilly added that the Tor Project regularly meets with law enforcement agencies to explain to them how Tor works and to point them towards known non-Tor-specific server vulnerabilities.

“Hidden services are just like a street address,” she wrote in an e-mail to Ars on Tuesday. “You can't break an address. You can break the doors or windows of the house at that address. An attack on a .onion and a .com are the same. The usual PHP vulnerabilities to SQL injection and the like are applicable.”

Anonymous used similar tactics in October 2011 when it exposed the members of another Tor-enabled child porn ring.

The newly published document is a Complaint/Assessment Form filed by a citizen who approached the Detroit office spanning June 2011 to August 2011. It details an investigation of the “Silk Road,” a Tor-enabled site that trafficked in illegal drugs. The complainant describes in the form that while investigating the site, he came across an adult section on a different hidden service, and separate from the Silk Road. There, the complainant encountered pictures that “looked like child pornography because he could tell the subjects were very young with some in diapers. All were still images, no videos, and he said most showed the children posing for the pictures.”

The “complainant” said he had wiped the hard drive, but showed his supervisor the computer that he had used, and that he had no other information about the subjects of the pictures or where they came from. On August 2, 2011, the assessment was closed.

Update: Jenny Shearer, an FBI spokesperson, e-mailed Ars with this comment: "The FBI’s Innocent Images National Initiative investigates the online sexual exploitation of children, to include photos of minors on the Internet. We take complaints about the online sexual exploitation of children seriously and act appropriately. The FBI does not comment on allegations reported to us by the public, nor do we comment on the methods and techniques we use. Tips received from the public are thoroughly analyzed for potential investigative follow-up."