A federal appeals court said the US Navy's scanning of the public's computers for images of child pornography constituted "a profound lack of regard for the important limitations on the role of the military in our civilian society."

The Naval Criminal Investigative Service (NCIS) practice led the 9th US Circuit Court of Appeals to suppress evidence in the form of images of child pornography that an NCIS agent in Georgia found on a Washington state civilian's computer. The agent was using a law-enforcement computer program called RoundUp to search for hashed images of child pornography on computers running the file-sharing network Gnutella.

"...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," Judge Marsha Berzon wrote for the San Francisco-based appeals court.

The court ruled 3-0 Friday that the Obama administration's position on the case would render "meaningless" the Posse Comitatus Act (PCA), which largely prohibits the military from enforcing civilian law. The PCA was first passed in 1878.

Judge Berzon wrote (PDF):

The government's position that the military may monitor and search all computers in a state even though it has no reason to believe that the computer’s owner has a military affiliation would render the PCA’s restrictions entirely meaningless. To accept that position would mean that NCIS agents could, for example, routinely stop suspected drunk drivers in downtown Seattle on the off-chance that a driver is a member of the military, and then turn over all information collected about civilians to the Seattle Police Department for prosecution.

The facts of the case aren't disputed. Acting on a tip from the Navy, the local Algona Police Department obtained a warrant to search defendant Michael Dreyer's computer in 2010. Dreyer was eventually convicted and sentenced to 18 years for possessing and distributing online child pornography. If it stands, the new decision means that a lower court is bound to vacate the conviction and sentence.

"This is, literally, the militarization of the police," Dreyer's attorney, Erik Levin, told the San Francisco Chronicle. "They have enough funding that they can go out and stray from the core mission of national security and get into local law enforcement."

Judge Andrew Kleinfeld wrote in a separate, concurring opinion:

The Navy did not just peek into Dreyer's home computer. It peeked into every computer in the State of Washington using the peer-to-peer file sharing program, "Gnutella." That is more "widespread" than any military investigation of civilians in any case that has been brought to our attention. Millions of people use Gnutella, and millions of people live in Washington, so the number of Gnutella users in Washington is likely quite large. As for being "repeated," the Posse Comitatus violation was repeated against every Gnutella user in Washington.

Kleinfeld concluded that "[l]etting a criminal go free to deter national military investigation of civilians is worth it."

Judge Diarmud O'Scannlain wrote in a separate opinion that Dreyer's conviction should stand, even if the military overstepped its boundaries. O'Scannlain said, "I respectfully dissent from the majority's misbegotten remedy for that violation."

He wrote that there was "meager evidence of PCA violations contained in the record," and the "violation at issue here does not merit application of the exclusionary rule" to "the benefit of a convicted child pornographer."