Who it did interview

"The FA is also facing new questions about who it did interview after sources within the England set-up revealed their information was that none of the players in the relevant meeting – not only Spence – were asked by either inquiry about what they had heard. The FA, presented with that evidence, has chosen not to deny it." (The Guardian.)

What does the pronoun "it" refer to in the prepositional phrase "about who it did interview..." in the passage above?