A federal appeals court is having second thoughts about its decision frowning on the US Navy for scanning every computer in the state of Washington accessing Gnutella, a large peer-to-peer network.

The September decision (PDF) thwarted a child pornography prosecution that began when a Naval Criminal Investigative Service agent in Georgia discovered the illicit images on a civilian's computer in Washington state. The agent was using a law-enforcement computer program called RoundUp to search for hashed images of child pornography.

Following the court's 3-0 decision, the Department of Justice petitioned for a rehearing. The 9th US Circuit Court of Appeals agreed Wednesday to revisit the dispute with a larger, 11-judge panel.

Further Reading Court blasts US Navy for scanning civilians’ computers for child porn

In September, a three-judge panel ruled that the military unlawfully intruded into civilian affairs. Allowing the prosecution to go forward, the court ruled, would render "meaningless" the Posse Comitatus Act (PCA). The PCA largely prohibits the military from enforcing civilian law, the court ruled. The PCA was first passed in 1878.

"...RoundUp surveillance of all computers in Washington amounted to impermissible direct active involvement in civilian enforcement of the child pornography laws, not permissible indirect assistance," the court concluded.

The case concerns a Washington state defendant named Michael Dreyer. Acting on a tip from the Navy, the local Algona Police Department obtained a warrant to search Dreyer's computer in 2010. Dreyer was eventually convicted and sentenced to 18 years for possessing and distributing online child pornography.

Dreyer's attorney called his client's prosecution "the militarization of the police."

The DOJ argued in its petition (PDF) for rehearing that the NCIS agents involved in the porn-scanning program are civilians not subject to "PCA-like restrictions." The evidence against Dreyer, the government added, should not be suppressed from the case "to deter" online child pornography.

Further, the DOJ also said in its petition that NCIS agents have the right to search peer-to-peer file-sharing sites just like anybody else.

"It amounted to nothing more than looking at files available to anyone seeking child pornography on a publicly available peer-to-peer network," the DOJ said.