A federal judge in Massachusetts ruled Wednesday in favor of a man accused of accessing child pornography through Tor, finding that the warrant issued by a Virginia-based judge was invalid. The evidence of child pornography the government claims it found on the man's computers is suppressed, which likely makes continuing prosecution of this case significantly more difficult.

That warrant, which was issued in early 2015, allowed federal investigators to use a "network investigative technique" (NIT), government-speak for a piece of malware typically used to penetrate the digital security of Tor users. That malware then led authorities to the computer of defendant Alex Levin, of Norwood, Massachusetts, and they criminally charged him with possession of child pornography.

Earlier this year, Levin’s lawyers challenged the judicial authorization to deploy the NIT.

"It allowed government agents to conduct a borderless dragnet search with no geographic limitation," J.W. Carney, Levin’s lawyer, wrote in a court filing. "Rule 41 simply does not permit a magistrate judge in Virginia to authorize the search of the defendant’s computer located in Massachusetts."

In a 39-page opinion issued on Wednesday, US District Judge William Young sided with Levin, saying that the Department of Justice and Congress had the authority to change the law—and was even currently attempting to do so.

For two years now, the DOJ has wanted to expand judges’ ability to sign off on the deployment of such malware. The change would give federal authorities an expanded ability to conduct "remote access" under a warrant against a target computer whose location is unknown or outside of a given judicial district. It would also apply in cases where that computer is part of a larger network of computers spread across multiple judicial districts. In the United States, federal warrants are issued by magistrate judges who serve one of the 94 federal judicial districts and are typically only valid for that particular jurisdiction.

Judge Young, however, also noted that federal rules of procedure and case-law do not restrict the ability of more senior judges, known as "district judges," to issue orders outside of their district. Among other citations, Judge Young cited a case that found "[u]nlike magistrates, the jurisdiction of district courts is usually defined by subject matter and parties rather than strictly by geography."

He concluded:

Based on the foregoing analysis, the Court concludes that the NIT Warrant was issued without jurisdiction and thus was void ab initio. It follows that the resulting search was conducted as though there were no warrant at all. Since warrantless searches are presumptively unreasonable, and the good-faith exception is inapplicable, the evidence must be excluded.

Ahmed Ghappour, a law professor at the University of California, Hastings, concurred with this reasoning.

"Judge Young applied sound reasoning in suppressing the evidence," he e-mailed Ars. "The federal rules of criminal procedure do not authorize magistrates to issue warrants for remote searches of targets whose location is unknown. The DOJ knew full well that the magistrate lacked authority to issue an out-of-district warrant, having proposed an amendment to the federal rules well over a year before the application was made in this case."

Neither Carney, nor the prosecutor in the case, David Tobin, immediately responded to Ars’ request for comment.