Snowden defended his expertise in portions of the interview that aired at 6:30 p.m. ET on Nightly News. The extended, wide-ranging interview with Williams, his first with a U.S. television network, airs Wednesday at 10 p.m. ET on NBC.

“I was trained as a spy in sort of the traditional sense of the word, in that I lived and worked undercover overseas — pretending to work in a job that I’m not — and even being assigned a name that was not mine,” Snowden said in the interview.

Snowden described himself as a technical expert who has worked for the United States at high levels, including as a lecturer in a counterintelligence academy for the Defense Intelligence Agency and undercover work for the CIA and National Security Agency.