One of the documents the EFF got its hands on is a memo from 2008, detailing a meeting with the FBI's Cyber Working Group at the Geek Squad's repair facility in Kentucky. During that meeting, the company even gave the feds a tour of the facility. Other documents showed that the two entities worked so frequently together, that they've developed a process for reporting suspicious content.
When a repair technician finds something they believe is child porn in a computer, they have to call the feds, and an FBI agent would arrive to inspect it. If the agent agrees that it's child porn, the PC or hard drive would be seized and sent to an FBI office nearest to the location of the device's owner. The agents there would then dig deeper and secure a search warrant if needed.
The FBI classifies the Geek Squad technicians who call in the reports as informants, but some of the documents in EFF's possession suggest they sometimes do more than report something they find. One shows that the feds paid at least one tech $500, and it's reportedly one of the payments involved in the child pornography case that compelled the EFF to file for a FOIA.
The EFF says that by paying informants, the agency is encouraging technicians to actively look for content. Case in point: the technician who called in the evidence for the California doctor's case reportedly found it in an unallocated space in his computer. That suggests that they didn't just stumble upon the evidence -- they used software with the intention of finding files that were already deleted. The EFF says the relationship between the agency and the retailer "potentially circumvents computer owners" Fourth Amendment rights, so it plans to go after the other documents the feds failed to produce for the FOIA it filed.