Peer-to-peer networks are better known for the trading of copyrighted songs and videos than the distribution of child pornography.

But Peter W. North, of Alexandria, a Department of Homeland Security subcontractor who federal agents believe is one of the nation’s most prolific disseminators of child pornography, allegedly used networks of personal computers to download and send his pornographic images.

In the early days, peer-to-peer networks like Napster collected digital files and stored information on them in centralized databases. But as Napster broke apart under a hail of lawsuits in 2001, new sharing systems formed in its wake.

Today, programs like LimeWire and BearShare, designed on the open-source code of Gnutella, use a decentralized system where files are stored on personal computers connected via the network, but the network provides no directory, which can make tracking illegally shared files more difficult, a federal agent said in a sworn statement.

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To seek out child pornography distributors, the federal Wyoming Internet Crimes Against Children Task Force searches peer-to-peer networks for the graphic images, downloads them and then uses law enforcement software to find other Internet users who also are sharing the file.

By reviewing the shared files, and coding embedded in each file, investigators are able to identify a pattern of activity linked to a single Internet Protocol address and in turn use that to find the person behind the file-sharing.

Agents tracking child pornography were led to 69.143.43.43, an IP address generated by Comcast. The cable company and Internet provider told federal agents the address belonged to North, according to the agent’s statement.

fklopott@dcexaminer.com