A massive FBI child porn probe has lawyers asking: Does the end justify the means?

Prosecutors describe Terry Lee Carlson as a "hands-on" sexual predator who boasted about abusing young boys as they slept — even filming some assaults. After FBI agents arrested him last year, the 47-year-old from Coleraine, Minn., said he needed help and wanted "things to be right."

Carlson is one of nearly 900 people arrested worldwide after a massive, controversial FBI takedown of the "dark web" child pornography site called Playpen.

But now he could play a central role in a major constitutional challenge to the FBI sting. This spring, in a decision closely watched across the country, a federal magistrate judge in Minneapolis recommended that evidence seized from Carlson's home, including 20 electronic devices storing homemade videos, be tossed out because a search warrant issued in Virginia wasn't enough to permit the FBI to deploy a hacking technique on Playpen visitors' computers anywhere in the world.

U.S. Magistrate Judge Franklin Noel ruled favorably on a motion from Carlson's attorney, Lee Johnson, finding that the FBI's use of a computer search technique — described by some as a form of malware — on any computer accessing Playpen was an unconstitutional search.

The opinion is the first by a Minnesota jurist among more than 50 challenges to cases tied to the FBI's "Operation Pacifier." The district's chief judge, John Tunheim, will decide in coming weeks whether to adopt Noel's ruling.

Noel's opinion offered harsh words for the FBI's Network Investigative Technique (NIT) and the investigators' conduct, which included keeping a copy of Playpen running for more than two weeks after arresting the site's creator.

"The purpose and flagrancy of the FBI's misconduct in attempting to obtain the NIT warrant and deploying the NIT malware is truly staggering," Noel wrote in March, adding: "In essence, the FBI facilitated the victimization of minor children and furthered the commission of a more serious crime."

Noel's opinion has rankled some in the law enforcement community, and Assistant U.S. Attorney Carol Kayser argued that the government's copy of Playpen merely allowed users to post links to video files with preview images that would have still circulated elsewhere.

The FBI called Playpen one of the world's largest child pornography websites, with more than 150,000 users. Users would access Playpen via Tor, an encrypted network that allows users to hide their location — a cloak that required investigators to be creative in tracking down other Playpen users.

The probe led them to a handful of Minnesotans like Carlson, who is now awaiting trial on 11 child pornography charges that carry the possibility of decades in prison.

Colin Fieman, a federal public defender in Tacoma, Wash., who created a national working group of more than 100 attorneys who follow Pacifier case filings, said Carlson's case is one of the first to successfully challenge the constitutionality of the Virginia warrant and the actions of law enforcement.

"I always thought [prosecutors] were fighting a losing battle in terms of trying to reconfigure the warrant and its arguments," Fieman said.

But few cases have had evidence suppressed, and while multiple judges have raised procedural questions about the warrant, many have upheld evidence on other grounds, Fieman said.

In the Carlson case, Noel rejected the government's defense of using the Virginia search warrant, saying it "does not identify which computers will be searched until the search is actually completed."

In an April 24 filing objecting to Noel's opinion, Kayser called his determination that the FBI acted recklessly "untenable." And no other court, she said, has found that the warrant's scope could not extend beyond the Eastern District of Virginia.

"When a warrant authorizes a search of many child pornography offenders' computers, that fact does not make the warrant constitutionally infirm," Kayser wrote. "Rather, it is a troubling indication of how prevalent these crimes are in our society."

Several Pacifier cases are making their way through federal appellate courts and could ultimately reach the Supreme Court. A December 2016 rule change now allows magistrate judges to issue warrants for searches outside their districts in some cases, where evidence is "concealed through technological means," but attorneys say constitutional concerns remain.

"We have appropriate and time-tested Fourth Amendment limits" on such technologies," said Mark Rumold, senior staff attorney at the nonprofit Electronic Frontier Foundation (EFF). "Unfortunately in this case, the FBI did not do that — they just threw caution to the wind and got the broadest authority that they could conceive of. It's going to have consequences down the line."

At least three other Minnesota cases have been linked to Operation Pacifier, all three resulting in quick plea agreements.

Robert Richman, an attorney whose client was sentenced to 2½ years in prison last month, said the government netted more people like his client than offenders like the site's operators, who have been sentenced to up to 30 years in prison. Like Noel, Richman argues that the government temporarily became the country's "largest purveyor" of child porn to win those convictions.

"There are places where the ends simply do not justify the means," Richman said.

Stephen Montemayor • 612-673-1755

Twitter: @smontemayor