For years, with seemingly little to no oversight, the Naval Criminal Investigative Service (NCIS) has been monitoring vast amounts of non-military U.S. Internet traffic and communications, looking for evidence of criminal activity. Last year, the U.S. Court of Appeals for the Ninth Circuit correctly held this “extraordinary” and illegal surveillance violated the Posse Comitatus Act ("PCA") and suppressed evidence gathered by NCIS on a civilian.

In an amicus brief we filed today with the ACLU of Washington and the National Association of Criminal Defense Lawyers, we argue that the court got it right.

In United States v. Dreyer, a Naval Criminal Investigative Service (“NCIS”) officer stationed in Georgia believed he was entitled to conduct Internet surveillance of any computer within a specific jurisdiction and did not have to limit his monitoring to U.S. military or government computers or personnel. While monitoring computers in the state of Washington, he identified an IP address sharing child pornography, determined the IP address belonged to Dreyer, and passed that information along to local police who arrested Dreyer.

The problem? Dreyer, like most residents of Washington, was a civilian and had no connection to the military. The Posse Comitatus Act (“PCA”), a federal statute enacted in 1876, prohibits the military from investigating civilians and otherwise participating in civilian law enforcement activities.

In the district court, Dreyer unsuccessfully challenged the NCIS surveillance as a violation of the PCA, but a three-judge panel of the Ninth Circuit reversed. Noting this wasn’t the first time the NCIS had engaged in this sort of activity, the appellate court found the surveillance “extraordinary” and in clear violation of the PCA. Most importantly, the panel found that the evidence NCIS obtained should be suppressed. This is the first time a federal court has suppressed evidence obtained in violation of the PCA (a few state courts have suppressed PCA violations). The government asked the court to rehear the case but abandoned any argument that there was a PCA violation. Instead, the government merely urged the court to reconsider its decision to suppress the evidence. The Ninth Circuit agreed to reconsider, deciding to have the case argued again before an 11-judge en banc panel.

Our amicus brief explains why the three-judge panel’s opinion should stand and the evidence should be excluded. The PCA’s passage stems from a long tradition in the U.S. of keeping the military out of civilian affairs, a tradition reflected in the structure of government laid out in the U.S. Constitution and in the Bill of Rights. Federalism generally gives states, rather than the federal government, the power to police civilians subject to limited exceptions. Separation of powers divides military power between the executive and legislative branches, where the president is commander in chief of armed forces, but Congress is responsible for maintaining and regulating the military. And the Third Amendment, which prohibits housing soldiers in private homes, reflects a desire to preserve the privacy of the home from military intrusion.

The PCA’s legal protections are crucial to preserving the important constitutional limitations on military involvement in civilian activities. While the military should know these limits, as we’ve uncovered through Freedom of Information Act requests, it has repeatedly conducted improper civilian surveillance. That includes U.S. Army-issued National Security Letters, a honey pot established by the Air Force that violated the Foreign Intelligence Surveillance Act (“FISA”) and an order of the Foreign Intelligence Surveillance Court (“FISC”), and Army Cyber Counterintelligence officers covertly attending the Black Hat computer security conference without proper authorization.

While the PCA is a criminal statute, it appears the government has never charged anyone with violating it since it passed 136 years ago. Given the history of improper military excursions into civilian affairs, the expansive Internet surveillance that occurred here, and the fact technological advancements make it easier for the military to conduct widespread Internet surveillance, the only way to deter military officials from intruding into civilian affairs is to exclude evidence it improperly obtains.

The en banc Ninth Circuit is scheduled to hear oral argument the week of June 15 in San Francisco.