A federal judge is scheduled to hear arguments on that request Friday.
Who decides that this is the best method of identifying these people? 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.
During that time, federal prosecutors told defense lawyers that the site included more than 23,000 sexually explicit images and videos of children.
Federal agents first noticed Playpen not long after it went online in August, 2014.
Some of the images described in court filings involved children barely old enough for kindergarten.
But they have also prompted a backlash of a different kind.
The Justice Department said in court filings that agents did not post any child pornography to the site themselves.