When a report surfaced earlier this week of a virus capable of implanting child pornography on the computers of unsuspecting people, it wasn't news to Capt. Rhett McQuiston.

"We've heard this story before," said McQuiston, who supervises the Internet Crimes Against Children Task Force for the Utah Attorney General's Office.

"It's kind of an old story with a new twist that's making its rounds through the media right now," the captain told the Deseret News.

On Monday, the Associated Press reported that it had uncovered cases across the country of innocent people being labeled pedophiles after someone found pornographic images of children that had been placed on their PCs by a virus. The report also raised the possibility that pedophiles might exploit infected computers to remotely store and view their explicit images or videos without fear of detection.

"We've had individuals claim that up front," McQuiston said. "Then as the investigation goes on, we've been able to prove otherwise."

That proof comes in many different forms, often including a suspect's confession during a police interview. But McQuiston said it typically comes from an extensive forensic examination of the individual's computer that involves, among other things, a review of visits to known child pornography Web sites, the quantity and number of times saved images or videos were accessed by the user, and keywords entered into online search engines.

"We put as much effort into proving that the behavior is going on as we do into proving that it is not going on," McQuiston said. "There's a lot that goes into these investigations. We've never once just gone off the images alone and said we're taking someone to jail."

After being contacted by the Deseret News, McQuiston said he spoke with the individual responsible for conducting more than 300 forensic computer investigations for the attorney general's task force. McQuiston said the technician told him he had yet to encounter what the captain called the "mystery virus" that was cited in the AP article.

"I'm not saying it doesn't exist," he said. "I'm saying it's one of the things we thoroughly check for because we know it's a defense that's been used in the past."

Joseph Jardine, a Salt Lake criminal defense attorney, said the specter of a child pornography virus poses a real threat to the public, especially in Utah where so many home computers have only one login and password but are used by multiple people.

"It's a problem because you're now going to have child porn images showing up on the computers of innocent people," Jardine said. "And depending on the forensics that are used, it may or may not be detected that a virus was responsible for it."