Snowden documents leaked to Der Spiegel in December 2013 describe a different type of NSA program from the sort that is usually publicized.

Rather than revealing software developed by the agency in order to access computers, these revelations describe a secret elite hacking unit, dubbed Tailored Access Operations, or TAO.

FRONTLINE asked security expert Bruce Schneier to break down its capabilities:

The NSA is thought to deploy the TAO unit for specific hard-to-get targets — for example, to hack systems, tap cellular phone networks, or intercept routers and servers at shipping ports — in order to implant them with surveillance devices.

Still, the extent of the unit’s capabilities remain somewhat unknown.

TAO is apparently a mystery even to many NSA employees, who require special security clearance to get access to its command center at the NSA’s headquarters in Fort Meade, Maryland.