Court documents suggest the FBI paid a Geek Squad technician who located images of child abuse on a customer’s computer $500 (£412).
Companies that carry out computer repairs are legally required to report such images, but a warrant is required for actively searching for material.
Defence lawyers argue that the FBI was directing the technician to look for illegal activity.
Geek Squad’s parent company, Best Buy, and the FBI have denied any wrongdoing.
Defence lawyers claim that the FBI had had eight “confidential human sources” in the Geek Squad, according to the Washington Post, which first reported the story.
The case has led online tech publication Network World – which declared the practice “unconstitutional” – to call for users to boycott Best Buy.
The case stretches back to November 2011 when an image of a young naked girl was found on the hard drive of Californian doctor, Mark Rettenmaier.
His lawyers want the case thrown out, arguing that the image was gleaned from an illegal search.
The FBI claim that a later search of Dr Rettenmaier’s iPhone found 800 images of naked girls.
Defence lawyer James Riddet claimed in a court filing last month that “the FBI was dealing with a paid agent inside the Geek Squad who was used for the specific purpose of searching clients’ computers for child pornography and other contraband or evidence of crimes”.
He wants to examine the relationship between the FBI and the technician – Justin Meade – in court, in a hearing scheduled for August.
In a statement, Best Buy spokesman Jeff Shelman said: “Best Buy and Geek Squad have no relationship with the FBI.
“From time to time, our repair agents discover material that may be child pornography and we have a legal and moral obligation to turn that material over to law enforcement.
“We are proud of our policy and share it with our customers before we begin any repair.”
He added: “Any circumstances in which an employee received payment from the FBI is the result of extremely poor individual judgment, is not something we tolerate and is certainly not a part of our normal business behaviour.”
Court filings show the FBI also denies the claims.
“I never asked or ordered Mr Meade or any Best Buy employee to search for child pornography or gather information on child pornography or any other crimes on my behalf or on behalf of the FBI,” Tracey Riley, the FBI agent Mr Meade contacted, wrote in a declaration.
The case began when Dr Rettenmaier took his computer to a Best Buy in November 2011 after it failed to boot up.
The hard drive was later shipped to Geek Squad’s maintenance centre and in January 2012, Mr Meade contacted Ms Riley to say a technician had found something suspicious.
The Geek Squad had to use specialised tools to recover the photos because they were either damaged or had been deleted, according to court papers.
Defence lawyers argue that it is impossible to tell when the files were placed on the hard drive or who accessed them.
According to defence lawyers, an informant file they received from prosecutors suggested:
- Mr Meade’s relationship with the FBI had begun in 2007
- He had been officially signed up as a source for investigations into images of child sex abuse in 2009
- In 2010 and 2011 he had contacted the FBI more than a dozen times
They also alleged that the FBI had paid him $500 for work between October 2010 and September 2011.
If it is proved that Mr Meade was working on behalf of the FBI, the case could be thrown out, according to law experts.
“If the government wants to look at somebody’s computer, they need to get a warrant,” UC Irvine Law School dean Erwin Chemerinsky, told the LA Times.