The NSA has used secret technology to input and alter data in computers even if they are not connected to the Internet, according to NSA documents provided by Edward Snowden, computer experts, and American officials, The New York Times revealed Tuesday.

The technology uses radio signals generated in tiny circuit boards and inside USB cables inserted surreptitiously into the computers by a spy, manufacturer, or unwitting user, and transmitted covertly to a briefcase-size relay station that intelligence agencies can set up miles away from the target, the Times said. The system can also insert malware into the computer.

It is only used on foreign targets and defensively, according to an NSA spokesperson.