The National Security Agency has implanted software in nearly 100,000 computers around the world that allows the United States to conduct surveillance on those machines and can also create a digital highway for launching cyberattacks. While most of the software is inserted by gaining access to computer networks, the N.S.A. has increasingly made use of a secret technology that enables it to enter and alter data in computers even if they are not connected to the Internet, according to N.S.A. documents, computer experts and American officials. The technology, which the agency has used since at least 2008, relies on a covert channel of radio waves that can be transmitted from tiny circuit boards and USB cards inserted surreptitiously into the computers. In some cases, they are sent to a briefcase-size relay station that intelligence agencies can set up miles away from the target.