United States v. Dreyer (9th Cir. Sept. 12, 2014) (2-to-1 vote); here are excerpts from the majority, the concurrence, and the dissent (some paragraph breaks added, some removed):

A special agent of the Naval Criminal Investigative Service (NCIS) launched an investigation for online criminal activity by anyone in the state of Washington, whether connected with the military or not. The agent found evidence of a crime committed by a civilian in the state and turned it over to civilian law enforcement officials. The civilian, Michael Dreyer, was prosecuted, convicted, and sentenced to eighteen years in prison. We hold that the NCIS agent’s investigation constituted improper military enforcement of civilian laws and that the evidence collected as a result of that investigation should have been suppressed….

The Posse Comitatus Act (PCA), 18 U.S.C. § 1385, “prohibits Army and Air Force military personnel from participating in civilian law enforcement activities.” [Footnote: The PCA states, “Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.”] We have previously recognized that, “[a]lthough the PCA does not directly reference the Navy,” “PCA-like restrictions” apply to the Navy as a matter of Department of Defense (DoD) and Naval policy….

The government maintains that, even though PCA-like restrictions apply to the Navy, they do not apply to civilian NCIS agents…. [But] “… while DoD personnel may participate in civilian law enforcement activities in their private capacities, they may not do so under the auspices of the military.” …

More fundamentally, the government’s assertion that there is a meaningful difference between civilian and other employees of the Navy for the purposes of the PCA-like restrictions is unsound. The DoD policies have consistently proclaimed that they set forth “restrictions on participation of DoD personnel in civilian law enforcement activities.” They do not limit their reach to non-civilian personnel only. And any contention to the contrary is belied by the abundantly clear expressions in the most recent regulations and policy instructions. Both state that they “[a]ppl[y] to civilian employees of the DoD Components,” and that their restrictions on direct participation in civilian law enforcement “apply to all actions of DoD personnel worldwide,” with “DoD personnel” defined to include “Federal military officers and enlisted personnel and civilian employees of the Department of Defense.” …

The regulations and policies implementing the PCA and § 375 “generally prohibit ‘direct’ military involvement in civilian law enforcement activities but permit ‘indirect’ assistance such as the transfer of information obtained during the normal course of military operations or other actions that ‘do not subject civilians to [the] use [of] military power that is regulatory, prescriptive, or compulsory.’” Prohibited direct assistance includes “[u]se of military personnel for surveillance or pursuit of individuals, or as undercover agents, informants, investigators, or interrogators.”

We have “set forth three tests for determining whether military involvement in civilian law enforcement constitutes permissible indirect assistance: ‘[1] The involvement must not constitute the exercise of regulatory, proscriptive, or compulsory military power, [2] must not amount to direct active involvement in the execution of the laws, and [3] must not pervade the activities of civilian authorities.’” “If any one of these tests is met, the assistance is not indirect.”

Agent Logan’s 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. He acted as an investigator, an activity specifically prohibited as direct assistance….

Also, Agent Logan engaged in his investigation not in any support capacity to civilian law enforcement, but rather as an independent actor who initiated and carried out this activity. His actions thus were not “incidental” to the overall investigation into Dreyer, or limited to backup support. The results of his investigation served as the primary basis for the state search warrant. [Local Police Department] Officer Schrimpsher conducted no significant additional investigation before procuring the warrant — he only verified that Dreyer lived at the address he was given and that the descriptions that Agent Logan provided of the files seemed to describe child pornography. Without Agent Logan’s identification of Dreyer, his computer, and the child pornography on his computer, there would have been no search and no prosecution….

Accordingly, Agent Logan’s actions amounted to direct assistance to civilian law enforcement. The government nonetheless argues that Agent Logan’s investigation was proper because it falls into the “independent military purpose” exception to the prohibition on direct assistance.

The policies and regulations create “an exception to the general prohibition on direct involvement where” there is “an independent military purpose,” that is, “where the military participation is undertaken ‘for the primary purpose of furthering a military or foreign affairs function of the United States, regardless of incidental benefits to civilian authorities.’” Such military activities include “[i]nvestigations and other actions related to enforcement of the Uniform Code of Military Justice.” …

But Agent Logan’s search was not reasonably focused on carrying out such a legitimate military investigation. NCIS is authorized to investigate criminal operations that “significantly affect the naval establishment.” Agent Logan understood that he did not have the authority to search any location, but had to limit his searches to areas where there was “a Department of Navy interest.” Yet, Agent Logan’s search did not meet the required limitation. He surveyed the entire state of Washington for computers sharing child pornography. His initial search was not limited to United States military or government computers, and, as the government acknowledged, Agent Logan had no idea whether the computers searched belonged to someone with any “affiliation with the military at all.” Instead, it was his “standard practice to monitor all computers in a geographic area,” here, every computer in the state of Washington….

Having held that Agent Logan’s investigation violated the restrictions on the use of the military to enforce civilians laws, we consider whether suppression of the resulting evidence should have been ordered here. We have held that “an exclusionary rule should not be applied to violations of 10 U.S.C. §§ 371-378 until a need to deter future violations is demonstrated.” Such a need exists here, as there is evidence of “widespread and repeated violations” of these provisions.

The record here demonstrates that Agent Logan and other NCIS agents routinely carry out broad surveillance activities that violate the restrictions on military enforcement of civilian law. Agent Logan testified that it was his standard practice to “monitor[] any computer IP address within a specific geographic location,” not just those “specific to US military only, or US government computers.” He did not try to isolate military service members within a geographic area. He appeared to believe that these overly broad investigations were permissible, because he was a “U.S. federal agent[]” and so could investigate violations of either the Uniform Code of Military Justice or federal law.

The extraordinary nature of the surveillance here demonstrates a need to deter future violations. So far as we can tell from the record, it has become a routine practice for the Navy to conduct surveillance of all the civilian computers in an entire state to see whether any child pornography can be found on them, and then to turn over the information to civilian law enforcement when no military connection exists.

This is squarely a case of the military undertaking the initiative to enforce civilian law against civilians. “There must be an exceptional reason” to invoke the exclusionary rule for violation of posse comitatus-like regulations, and the broad use of military surveillance of overwhelmingly civilian populations is an exceptional reason….

That a need to deter future violations exists is further supported by the government’s litigation positions. The government is arguing vehemently that the military may monitor for criminal activity all the computers anywhere in any state with a military base or installation, regardless of how likely or unlikely the computers are to be associated with a member of the military. Such an expansive reading of the military’s role in the enforcement of the civilian laws demonstrates a profound lack of regard for the important limitations on the role of the military in our civilian society….

Kleinfeld, Senior Circuit Judge, concurring ….

Were we suggesting something like application of the exclusionary rule to all Posse Comitatus violations, then application of the exclusionary rule would be inappropriate. And if there were any reason to think that the violation in this case were a fluke, it would be inappropriate. This case, though, amounts to the military acting as a national police force to investigate civilian law violations by civilians.

Generally, the exclusionary rule does not apply to Posse Comitatus violations, in the absence of “widespread and repeated violations” demonstrating a need to deter future violations.In this case, unfortunately, that is just what we have. 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.” …

The military not infrequently investigates civilians or assists in civilian law enforcement incidentally to military law enforcement. A Navy shore patrol may break up a fight involving sailors at a waterfront saloon, and turn the civilians over to the local police. The Army may investigate a drug ring on base, and turn civilian spouses living on base and off- base civilian participants over to civilian authorities. There would be little reason to deter military law enforcement in cases like those, and indeed they would ordinarily not even be Posse Comitatus violations.

This case is different. There could be no bona fide military purpose to this indiscriminate peeking into civilian computers. It should be easy to distinguish this case from run of the mill military law enforcement that incidentally brings about apprehension of civilians.

True, the practical effect of the decision may be to let a criminal go. As Justice Cardozo wrote, application of the exclusionary rule means that “[t]he criminal is to go free because the constable has blundered.”

We are unlikely to see so widespread and repeated a Posse Comitatus violation from the Army or Air Force, because their military personnel would risk prison. If the military chooses to become a national police force to detect civilians committing civilian crimes, the Navy would be the branch to use, because the criminal penalty does not apply to Navy personnel. Without the criminal penalty, the exclusionary rule is about all that the judiciary has to deter such widespread and repeated Posse Comitatus violations as we have here. Letting a criminal go free to deter national military investigation of civilians is worth it.

O’Scannlain, Circuit Judge, concurring in part and dissenting in part ….

The exclusionary rule is “a judicially created remedy of [the Supreme Court’s] own making” whose “sole purpose” is “to deter misconduct by law enforcement.” … [E]xclusion is “our last resort, not our first impulse.” …

Despite [its] cost, application of the exclusionary rule might still be justified if there were evidence of “widespread and repeated violations” of the PCA. On this record, there is not.

The majority opinion primarily focuses on Agent Logan’s ostensible violations of the relevant regulations applying the PCA to the Navy. But the actions of one agent — no matter how egregious — do not show that violations are widespread. Agent Logan’s descriptions of his own practices are, therefore, of limited relevance.

Perhaps recognizing the thinness of this evidence, the majority opinion also points out that Agent Logan “began carrying out these searches with two other agents at least several months before he found Dreyer’s IP address.” It also references another supposed violation in Kentucky [citing a footnote omitted from the excerpt above -EV]. I fail to see how evidence that four agents committed violations — three of whom were part of the same investigative team — demonstrates a widespread problem. Such anecdotal evidence falls far short of what our precedents require before we will resort to the “extraordinary remedy” of exclusion, especially considering the cost of doing so in this case….

Finally, the majority opinion cites the government’s litigating position in this case as evidence of “a profound lack of regard for the important limitations on the role of the military in our civilian society.” Regardless of whether that observation is true, it does nothing to show that there have, in fact, been widespread and repeated violations of the PCA; it simply shows that the government wanted to win this case and put forward the best arguments it could to justify what Agent Logan did here. From the premise that the government believes it has a certain power, it does not follow that the government routinely exercises that power….

Thus, far from having “abundant evidence that the violation at issue has occurred repeatedly and frequently,” we have before us a paucity of evidence that does not come close to overcoming the “high obstacle for those urging application of the [exclusionary] rule.”

Thanks to How Appealing for the pointer.