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, according to people familiar with the probe, including his decision in May to fire FBI Director James B. Comey. Also of interest is the White House’s initial inaction after warnings about then-national security adviser Michael Flynn’s December discussions with Russia’s ambassador to the United States.