At least three Minnesota men have been charged with participating in a vast, secretive child pornography internet forum after being swept up in a far-reaching FBI sting considered the biggest hacking investigation in federal law enforcement history.

Justice Department officials say the FBI’s Operation Pacifier has helped build cases against roughly 200 people while identifying dozens of abused children and those who exploited them.

But Operation Pacifier has also triggered a series of legal challenges that are stirring constitutional debates over how law enforcement tries to smoke out criminals in the darkest corners of the web.

Federal judges in some states have thrown out evidence on grounds that the FBI shouldn’t have been allowed to hack private computers all across the country with a warrant from just one jurisdiction. Others remain concerned that the FBI actually committed a far worse crime — disseminating child porn — while investigating the case.

“There’s the stunning issue of the government becoming the country’s major purveyor of child pornography for a period of two weeks,” said Robert Richman, a Twin Cities attorney who has monitored Pacifier cases across the country since he began representing a local defendant earlier this year.

The sting began last year, when the FBI seized the computer server of a child porn site called Playpen. The site operated on the Tor network, a “dark web” browser where users’ activities are more easily obscured than on the regular World Wide Web.

3: Minnesotans charged, at least, in child porn sting. 200: Suspects charged nationally, roughly.

But rather than immediately take down Playpen — which had nearly 215,000 members — the FBI kept it running for two weeks, using malware to discover users’ locations.

The resulting constitutional questions are unlikely to be addressed in Minnesota any time soon: The first three men charged here chose to negotiate plea agreements rather than join at least 19 legal challenges around the country.

“If we were in Massachusetts we would have a pretty good precedent for ourselves, but we’re in Minnesota,” said Eric Nelson, a Bloomington attorney whose client faces mandatory prison after pleading guilty last month. “You don’t know how a court is going to look at it. … The individual has to weigh out the risks vs. the rewards.”

Ends justify means?

In April, agents searched the Brooklyn Park home of Nelson’s client, Christopher Todd Gordon, retrieving more than a dozen electronic devices. On Playpen, Gordon was “tanda16,” according to an FBI agent’s affidavit, an account used to access 87 hours of child pornography before the site was finally taken down.

Gordon quickly pleaded guilty to receipt of child pornography after being charged in August and faces five to 20 years in federal prison.

Defendants elsewhere have argued that searches in their cases, based on the same warrant issued in Virginia, violated the Federal Rules of Criminal Procedure by expanding the scope of the search beyond the district where a magistrate judge signed off in 2015.

Judges around the U.S. have splintered — some suppressing all evidence, others allowing evidence despite finding that the searches were unlawful, and still others taking no issue with the original warrant.

Colin Fieman, a federal defender in Tacoma, Wash., created a national “defense working group” of about 80 federal defenders and private attorneys that track and discuss the legal challenges. A judge sided with Fieman in one of his first cases after the FBI refused a court order to explain how it found a vulnerability in the Tor network, on which Playpen operated, and how it hacked into users’ electronic devices.

Fieman has also petitioned unsuccessfully for entire cases to be dismissed on grounds of “outrageous government conduct.” He conceded that winning such arguments is unlikely but said he thought it’s worth showing judges how the FBI conducted its operation. In those motions, Fieman argued that the government actually did a better job hosting a child porn site than whoever originally ran Playpen: The site’s average weekly visitors jumped from 11,000 to 50,000 during the 13 days it was controlled by the FBI.

“There’s no reasonable dispute that this was illegal,” Fieman said. “Apart from the dubious moral consequences of what they did, it’s really coming down to making the ends justify the means.”

‘This is the cutting edge’

Absent congressional intervention by Dec. 1, a change to Rule 41 of the Federal Rules of Criminal Procedure will allow federal law enforcement agencies to search for suspects concealing their identities online based on a single warrant. Justice Department officials have said this would help them catch child exploitation that is otherwise hard to detect.

“The … anonymity that they achieve there makes it very, very difficult for law enforcement to identify them,” one official said. “We don’t want … to rest while children are the ones that are sort of being sacrificed or who are suffering abuse based on the perception of the safe haven.”

Though the suspects charged in Operation Pacifier don’t cut a sympathetic profile, civil liberties advocates worry about the precedent the cases could set.

“We are approaching a bad situation where the courts don’t understand the surveillance technologies they are being asked to oversee,” said Christopher Soghian, principal technologist and senior policy analyst at the American Civil Liberties Union. “We really do need to have a national debate about government hacking.”

The nonprofit Electronic Frontier Foundation (EFF) has also followed the cases closely around the country. Though it doesn’t consider hacking to be an inappropriate law enforcement tool by itself, the EFF worries about the scope of Operation Pacifier.

“It’s evidence of the need for greater restrictions on law enforcement using this type of hacking,” said Mark Rumold, senior staff attorney for the EFF. “That’s not what’s happening right now, which is the doors are going to be flung wide open and this is going to happen more and more.”

Many legal observers expect the debate to reach the Supreme Court one day. For his part, Richman has had to weigh the interests of his own client against a desire to argue civil liberties concepts likely to have lasting implications. After a recent legal analysis of the cases, Richman concluded that judges are typically ruling in favor of the prosecution.

“This is one of those cases that is a great issue for lawyers,” Richman said. “This is the cutting edge. This is the place where constitutional law has not caught up with changes in technology. That being said, it’s not necessarily a great case for clients.”

Twitter: @smontemayor