An appeals court rulinghas offered a rare glimpse at the extent to which military police investigations reach into civilians' computers. Apparently, they scan civilian computers quite often — and to a degree that a 9th Circuit appeals court has now found violates the 1878 Posse Comitatus Act.

Some background: Posse Comitatus bars the military from enforcing state laws. But each military branch is still allowed to police — and investigate — its members off-base. The Navy has the Naval Criminal Investigative Service, and one of its units based in Georgia spends its time looking for Navy personnel who trade in child porn online. One of those searches caught a civilian, Washington state resident Michael Dreyer. When NCIS saw that he wasn't in the military, its investigators passed the information on to civilian law enforcement, which led to Dreyer's arrest and prosecution.

And that's the problem, says the court. Military police aren't supposed to be making cases against civilians.

The problem is how they caught him. We're not talking about NSA-level espionage; this is more basic. NCIS is using software that scans file-sharing services such as Gnutella for illicit images. Civilian law enforcement agencies use the same kind of software, without the court's objection. If you open your computer up to file-sharing, it's pretty much guaranteed that the "shared" portion of your hard drive will be scanned by law enforcement — repeatedly.

The NCIS agent who caught Dreyer testified that he used the software to target file-sharers with IP addresses in the state of Washington, because it has Navy installations. But since there's no way to narrow that kind of search to Washington residents who are in the military, he was effectively searching the whole state.

The court likened that to having military police stop cars in downtown Seattle on the "off chance" that a drunken driver was in the military.

Dreyer's appellate attorney, Erik Levin, says the appeals court recognized that an important American legal principle was at stake.

"Part of the fabric of our culture is a deep-seated suspicion of the military engaging in civilian affairs," says Levin. "What [NCIS] did here is really unprecedented."

The decision to throw out Dreyer's 18-year sentence was not unanimous. One member of the three-judge panel, Diarmuid F. O'Scannlain, agreed that the NCIS has violated the Posse Comitatus Act, but he disagreed with what he called "the majority's misbegotten remedy for that violation."

"This case provides no justification for setting a convicted child pornographer free," O'Scannlain wrote.

This case may seem like a one-off situation, but it's not. Members of the U.S. military live online just as much — or more — than the rest of us, and military police are there watching them. Levin says the Georgia NCIS agent admitted that his unit routinely searches computers across whole geographic areas, and he knows of at least one other case of a civilian who was prosecuted after being caught by a military search.

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