When 21-year-old Jennifer Dunham of Concord was arrested in February for allegedly having child pornography on her computer, her attorney professed her innocence and said she had no idea the images were there.

Attorney Charles Keefe offered two defenses: Someone else had used Dunham's computer, or Dunham had inadvertently and unintentionally received the images herself while downloading legitimate images over LimeWire, a file-sharing network that allows users to search other people's computers for a specific file or image and download it onto their computer.

In other words, someone could download something titled "Beach Boys video" from a computer in Texas and open it to discover images of child pornography.

It remains to be seen how Dunham's case plays out. But computer forensic experts interviewed last week said Dunham would not be the first to fall victim to unintentional downloading.

"I've got to tell you, I feel sorry for that young lady," said Gordon Pelton, owner of Computer Forensics One in Northern California. "It is possible for that to happen."

Dunham, until recently a college student, was charged Feb. 26 with three counts of possession of child pornography following an investigation by the Concord police and the cyber crime unit at the state attorney general's office. The details of the investigation and the charges against Dunham are still not public, because the authorities have yet to file written charges against her in court.

Keefe, who also has not seen formal charges or an affidavit explaining the evidence against Duhnam, said he's been told that Dunham came to the authorities' attention this summer during a wide-ranging cyber crime investigation. When investigators discovered images of child pornography, they followed the transfer of those images to Dunham's computer in Concord, he said.

The authorities searched Dunham's home in late June, seizing computers and electronic evidence, according to the police. They also seized marijuana from the home, the police said. They analyzed the computer images and determined they were child pornography. In late February, Dunham turned herself in on three counts of possession of child pornography and one count of possession of marijuana.

She is scheduled to be arraigned Thursday in Concord District Court.

LimeWire and other peer-to-peer file-sharing networks have long come with security warnings for their users. But not only the sort raised in Dunham's case.

File sharing works this way: Users put all files, songs and images they are willing to share publicly into a folder on their computer. When someone in the network goes in search of a particular song, file or image, their computer will conduct a wide-ranging search of those shared folders. They will then get a list of what the search turned up and an option to download any of it.

A user can download a single file, song or image, or they can opt to download the entire contents of someone's folder. If that folder has child pornography, it's going to be downloaded too.

The risks are many and run in both directions: You may unknowingly import a virus into your computer. If you're not careful to make only parts of your computer files public, someone can gain access to everything in your system, even sensitive financial or personal data. And you can find yourself with illegal images you didn't intend to accept.

Susanne Nicholson, owner of Midwest Forensics in Illinois, said it is particularly difficult to know what you are taking from another person's files if his material is listed by file name and not by image. "Typically, (child pornography) files have very telling names," she said. "But not always."

That's because as the "typical" names of child pornographic images become known, users change the name to stay ahead of the authorities. Or perhaps, they are trying to hide the illicit images from someone else who shares their computer. Either way, that can make it even harder to know what you are downloading. (next page »)