Special counsel Robert S. Mueller III has alerted the White House that his team will probably seek to interview six top current and former advisers to President Trump who were witnesses to several episodes relevant to the investigation of Russia’s meddling in the 2016 election, according to people familiar with the request.

Mueller’s interest in the aides, including trusted adviser Hope Hicks, former press secretary Sean Spicer and former chief of staff Reince Priebus, reflects how the probe that has dogged Trump’s presidency is starting to penetrate a closer circle of aides around the president.

Each of the six advisers was privy to important internal discussions that have drawn the interest of Mueller’s investigators, including his decision in May to fire FBI Director James B. Comey and the White House’s initial inaction following warnings that then-national security adviser Michael Flynn had withheld information from the public about his private discussions in December with Russia’s ambassador to the United States, according to people familiar with the probe.

The advisers are also connected to internal documents that Mueller’s investigators have asked the White House to produce, according to people familiar with the special counsel’s inquiry.