One of the news articles the Wikipedia article cites [1] refers to Snowden explicitly as a "former CIA employee."
[1] http://articles.chicagotribune.com/2013-06-09/news/chi-nsa-p...
However, he says he worked as a systems administrator, which I don't think is a "CIA agent" - I believe (although I'm not 100% certain) that "agent" refers specifically to a spy.