Law enforcement has long complained that online services like Tor create a type of safe haven for criminals because they hide the unique network addresses from which people connect to sites on the internet.
But they have also prompted a backlash of a different kind.
Officials acknowledged those risks, but said they had no other way to identify the people accessing the sites.
The Justice Department said in court filings that agents did not post any child pornography to the site themselves.
Some of the images described in court filings involved children barely old enough for kindergarten.
Who decides that this is the best method of identifying these people?.
Federal agents first noticed Playpen not long after it went online in August, 2014.
Those hacks, developed with the help of outside contractors, were a technical milestone.