The theft raises the embarrassing prospect that for the second time in three years an insider has managed to steal highly damaging secret information from the N.S.A. In 2013, Edward J. Snowden, who was also a contractor for the agency, took a vast trove of documents that were later passed to journalists, exposing N.S.A. surveillance programs in the United States and abroad.
The information believed stolen by this contractor — who like Mr. Snowden worked for the consulting firm Booz Allen Hamilton, which is responsible for building and operating many of the agency’s most sensitive cyberoperations — appears to be different in nature from Mr. Snowden’s theft.
The contractor arrested in recent weeks is suspected of taking the highly classified “source code” developed by the agency to break into computer systems of adversaries like Russia, China, Iran and North Korea. Two officials said that some of the information the contractor is suspected of taking was dated.