Story

Courtesy of Ars today: http://arstechnica.com/tech-policy/2014/05/houseguest-downloads-child-porn-cops-show-up/

Local police in Marin communities like Novato are members of the regional Internet Crimes Against Children (ICAC) task force, and as such they participate in the common law enforcement practice of monitoring peer-to-peer file-sharing networks for possible child pornography files. In September 2013, Novato detective Amy Yardley was looking for such files being traded from Marin County IP addresses, and she scored a hit on the Ares network with a suspicious file downloaded by a Sausalito Internet subscriber.
Yardley passed the tip to the Sausalito Police Department, where detective Brian Mather obtained a search warrant for the subscriber's address. He showed up at the house with a search team but couldn't find any child pornography within. The home's residents, no doubt unnerved by both the search and the charge behind it, pleaded their innocence and gave Mather a complete list of all houseguests who had used their wireless network in recent months.
Investigating this list took months, but Mather eventually developed a suspect: Mark Magner, age 32, from nearby San Rafael. Police then searched Magner's home and seized his computer. A forensic examination of the machine turned up "multiple videos and pictures that depicted juveniles and children involved in sexual acts," in the words of a police department press release .