The mystery of the whereabouts of Edward Snowden's long-time girlfriend is solved in a documentary that premiered in New York on Friday night: she has been living with the national security whistleblower in Russia since July.

The surprise revelation in the documentary, filmed by Laura Poitras, upends the widespread assumption that Snowden had deserted Lindsay Mills and that she, in a fit of pique, fled Hawaii where they had been living to stay with her parents in mainland US.

Since Snowden, a former NSA contractor, outed himself last year as being behind the biggest leak in US intelligence history, Mills has remained silent, giving no interviews or any hints of her feelings on the subject of her boyfriend or his actions.

The two-hour long documentary, Citizenfour, shows Mills living in Russia with Snowden.