The investigative journalist Glenn Greenwald has found a second leaker inside the US intelligence agencies, according to a new documentary about Edward Snowden that premiered in New York on Friday night.

Towards the end of filmmaker Laura Poitras’s portrait of Snowden – titled Citizenfour, the label he used when he first contacted her – Greenwald is seen telling Snowden about a second source.

Snowden, at a meeting with Greenwald in Moscow, expresses surprise at the level of information apparently coming from this new source. Greenwald, fearing he will be overheard, writes the details on scraps of paper.

The specific information relates to the number of the people on the US government’s watchlist of people under surveillance as a potential threat or as a suspect. The figure is an astonishing 1.2 million.

The scene comes after speculation in August by government officials, reported by CNN, that there was a second leaker. The assessment was made on the basis that Snowden was not identified as usual as the source and because at least one piece of information only became available after he ceased to be an NSA contractor and went on the run.

The two-hour documentary was the highlight of the New York Film Festival.