By Nate Raymond

(Reuters) - A federal judge on Wednesday threw out the evidence against a Massachusetts man charged as part of an FBI investigation in which it secretly ran one of the largest child pornography websites on the Internet in order to catch its users.

The ruling by U.S. District Judge William Young in Boston marked a potentially major setback to a probe that has resulted in at least 137 people being charged in the United States and prompted overseas investigations.

Young ruled that a Virginia-based federal magistrate judge had no jurisdiction to issue a search warrant used to justify gathering evidence on Alex Levin, of Norwood, Massachusetts.

"It follows that the resulting search was conducted as though there were no warrant at all," Young wrote. "Since warrantless searches are presumptively unreasonable, and the good-faith exception is inapplicable, the evidence must be excluded."

A Justice Department spokesman declined to comment. Lawyers for Levin, who was arrested for possession of child pornography in August, did not respond to requests for comment.

In February 2015, the Federal Bureau of Investigation seized the server hosting Playpen, a child porn website operating on the Tor network, which is designed to facilitate anonymous online communication and protect user privacy.

In order to identify its 214,898 members, authorities sought a search warrant from the Virginia judge allowing them to deploy a "network investigative technique."

That technique, or malware, would cause a user's computer to send them data any time that user logged onto the website while the FBI operated it for two weeks.

Critics of the investigation have questioned the ethics of the FBI's decision to effectively become a child pornography distributor itself to catch potential offenders, even if briefly.

In his ruling, Young said he expressed no opinion on the FBI's tactic.

But he said the probe differed from undercover stings where the government buys drugs to catch the dealers, saying it was instead "something akin to the government itself selling drugs to make the sting."

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His ruling instead focused on jurisdictional issues. Levin's lawyers said the Virginia judge behind the warrant that allowed the FBI to transmit computer code to the website's users had no authority to authorize searching Levin's out-of-state computer.

Prosecutors argued that the warrant properly authorized the search as the server was in Virginia. But Young said that was "immaterial, since it is not the server itself from which the relevant information was sought."





(Reporting by Nate Raymond in New York; Editing by Simon Cameron-Moore)