Government officials, who would speak of the classified details of the case only on condition of anonymity, said that Mr. Pho took the classified documents home to help him rewrite his resume. But he had installed on his home computer antivirus software made by Kaspersky Lab, a top Russian software company, and Russian hackers are believed to have exploited the software to steal the documents, the officials said.
It is not clear whether anyone at Kaspersky Lab was aware of the document theft. The company has acknowledged finding N.S.A. hacking software on a customer’s computer and removing it, but says the material was subsequently destroyed. It has denied that it works with Russian intelligence. Nghia H. Pho, a software developer for the intelligence agency, admitted taking secrets that Russian hackers then stole from his home computer.