(TNS) -- They knew it as Playpen, a hidden website for people, like them, who wanted to view and share child pornography.
Thousands of people from around the world logged into the site from the privacy of their own homes or offices, to exchange messages and download lurid videos and photos of children.
What they didn't know was that federal agents had taken over the site and set up a "watering hole" sting to track down child predators.
The resulting sweep, dubbed Operation Pacifier - one of the largest the FBI has conducted on the elusive portion of the Internet known as the dark web - led to the arrest of more than 135 people in 18 states on child pornography charges, including seven men charged in Houston federal court.
Many of those arrested in Texas and elsewhere were white-collar professionals, including a pediatrician, a professor, a former bank executive and a federal drug enforcement agent. The FBI also arrested a public school administrator in Vancouver, Wash., a math teacher in Queens, N.Y., and a preschool teacher in central New Jersey.
Federal officials say the sweep, and others like it, are aimed at stopping victimization of children by people who share information anonymously.
"The perception of a safe haven for individuals that seek to exploit children is not something that we can tolerate, not something that this society can tolerate," one Department of Justice official noted in a briefing on the government's anti-pornography efforts, "so we have to pursue it, and we are, and have, and will continue to do so."
Many of those charged have pleaded guilty, including four of the Texas men arrested. Some defendants, however, are challenging the government's operation, saying federal officials violated their civil rights by operating the website for two weeks after seizing the server in North Carolina.
"Certainly, it's the first time that they've run a website of this magnitude," said Christopher Soghoian, principal technologist for the American Civil Liberties Union in Washington, D.C., who was called as a defense witness in one of the cases in western Washington state. "It's also the first time they've hacked this number of users successfully."
But the end doesn't justify the means, he said.
"The FBI facilitated the wholesale distribution of child porn for several weeks last year," he said. "It's truly sickening."
Obscured IP addresses
The Playpen sting began with a December 2014 tip to the FBI from law enforcement officials in an undisclosed foreign country, alerting authorities to a site with sexually explicit videos and images of children as young as infants.
Computer analysts at the FBI determined that Playpen had gone live in August 2014. The site operated on a Tor network - an acronym for "the onion router" - that sends visitors through multiple servers, much like the layers of an onion.
A Tor browser that is free to download protects the identities of visitors by hiding their Internet Protocol, or IP, addresses and keeps their movements anonymous.
A few months after receiving the tip, in February 2015, federal agents raided the home of a person believed to be the site administrator for Playpen in Naples, Fla. They seized the server, based in the foothills of the Blue Ridge Mountains in Lenoir, N.C., then moved it to a government facility in Newington, Va.
A U.S. magistrate judge in Virginia authorized a search warrant allowing federal agents to monitor the computers of anyone logging onto the site through use of a hacking tool - special malware - that allowed agents to seize the IP addresses from users' computers.
In a sworn statement, an agent told a federal judge in Washington that the Tor system had complicated the investigation so that agents needed two weeks, until March 4, 2015, to uncover users' identities.
The idea was to be on hand to nab anyone who stopped by anonymously, according to court records. And it worked.
Within those two weeks, Operation Pacifier identified more than 100,000 unique user accounts and approximately 1 million log-ons, though agents couldn't determine how many individuals the data represented, according to federal court documents.
Agents then filed subpoenas with Internet companies such as Comcast and Verizon to obtain the names and addresses of the people with broadband accounts at those IP addresses and later obtained search warrants to seize computers and thumb drives containing the illicit images.
So far, at least 135 people have been indicted for downloading Playpen content over the 13-day period, including 35 people accused of "hands-on" sexual offenses and 17 charged with producing child pornography.
Some defense attorneys are challenging the operation, saying the FBI tactics violated Fourth Amendment protections against unlawful searches and seizures.
Colin Fieman, a Tacoma-based federal public defender who represents a school administrator in southern Washington and other defendants caught up in Operation Pacifier, has argued in court that the FBI actively facilitated the uploading and redistribution of child pornography online.
The federal operation increased traffic to the site from 11,000 visits per week before the raid to 100,000 afterward, re-victimizing the children depicted in the images and videos, he argued.
And the FBI violated the scope of its search warrant, which he said was intended only for Playpen visitors who logged in within range of the Playpen server at the FBI site in Eastern Virginia, he said.
Soghoian, from the ACLU, said these arguments have never had much sway in court, however.
The ACLU is concerned that judges don't understand they're authorizing hacking software in the search warrants and that Congress has never given the FBI authorization to hack into people's home computers.
In the Western District of Washington case, the U.S. attorney's office stated in pleadings that the government's seizure and limited-time operation of Playpen using a court-authorized hacking tool was "necessary and appropriate" means to identify users logging around the country.
Playpen wasn't the first federal takeover of a child pornography site. The FBI began using hacking software in the early 2000s to track illicit activity in dark corners of the web.
The first pornography sting akin to the Playpen operation found 13,000 members in 2012 on sites with names like Pedoboard and Hard Candy, on servers based at a worksite and on home computers near Omaha, Neb.
Agents used hacking software to identify 25 people, of which 19 were indicted in several states. Among those arrested was Timothy DeFoggi, who had been acting director of cyber security at the U.S. Department of Health and Human Services. He was sentenced to 25 years in federal prison for engaging in a child exploitation enterprise.
Other site visitors were identified abroad, and more than a dozen children depicted in the images and videos were identified and rescued, Justice Department officials said.
In the latest case, the investigation reached into the Southern District of Texas, which stretches from the Houston area to Laredo. Of those charged, four defendants have pleaded guilty: the pediatrician, the bank executive, the professor and an IT specialist. Three others, including the agent for the U.S. Drug Enforcement Administration, are awaiting trial.
The Texas arrests were made as part of Project Safe Childhood through joint investigations by federal officials, local and university police, and the Texas Department of Public Safety, officials said.
Not all were white-collar professionals. Ken Magidson, U.S. attorney for the Houston region, said the seven defendants came from all walks of life.
But David Finkelhor, director of the Crimes Against Children Research Center at the University of New Hampshire, said there might be a reason so many professionals were caught up in the sting.
Finkelhor, a sociologist who has studied sex offenders for nearly four decades, said the vast majority who view child pornography are white males who skew toward a higher socioeconomic status than most criminals.
Federal perceptions may also be at work, he said.
"This particularly white-collar group may reflect some selection on the part of police," he said. "They have many more targets than they can investigate and tend to prioritize those in a position of authority, influence or access to children."
Officials anticipate more indictments as the federal Playpen investigation continues.
©2016 the Houston Chronicle. Distributed by Tribune Content Agency, LLC.