On September 8, 2017, it was reported that:
Special counsel Robert Mueller has reportedly told the White House that his team will seek to interview six senior and former aides to President Trump as part of the federal investigation into Russia's meddling in the 2016 presidential election. Among those Mueller's team is interested in interviewing are former White House press secretary Sean Spicer, former chief of staff Reince Priebus ...
Why would Mueller notify the White House that he wants to interview people who are no longer employed there? That seems unnecessary and would possibly give the White House an opportunity to contact those people and attempt to influence what they say.