Then-White House Chief of Staff Reince Priebus speaks at the Conservative Political Action Conference on Feb. 23, 2017 in Washington. (Bill O'Leary)

Reince Priebus, the former chief of staff to President Trump, was interviewed for a full day Friday by members of special counsel Robert S. Mueller's team, Priebus's lawyer said.

In a statement, William Burck said his client was interviewed voluntarily.

"He was happy to answer all of their questions," Burck said.

The interview, which took place at the special counsel's office in Washington, is a sign that Mueller's investigation is now reaching into the highest levels of Trump's aides and former aides.

[Mueller gives White House names of 6 aides he expects to question in Russia probe]

Priebus served as chairman of the Republican National Committee during the 2016 election campaign before joining the White House when Trump was inaugurated. He resigned as chief of staff in July after Trump had stewed for months about his handling of the White House's legislative agenda in the president's first months in office. He was replaced by John F. Kelly.

Mueller's team has also indicated an interest in interviewing a series of other current and former White House aides, including White House counsel Don McGahn and director of communications Hope Hicks.

[Mueller casts broad net in requesting extensive records from Trump White House]

Mueller has also requested White House documents about a series of topics, including Trump's firing of FBI Director James Comey and interactions with Michael Flynn, the former national security adviser who was fired over December conversations with then-Russian Ambassador Sergey Kislyak.

During his six months as Trump's top gatekeeper, Priebus oversaw a White House that was at times chaotic and freewheeling. Aides wandered in and out of important meetings, and Priebus sought to manage his often difficult boss by trying to be physically present with Trump as frequently as possible.

For that reason, Priebus was present at key moments that have drawn Mueller's interest, including as Trump worked to limit the growing questions about Russia's possible role in his election and in the discussions that led to Comey's firing.

Priebus, for instance, figured in one important interaction between Comey and Trump before the FBI director's dismissal.

Comey told Congress that during one Oval Office meeting in February, Trump shooed other aides from the room and then asked him about Flynn, who had recently been interviewed by the FBI and had been fired the day before.

"I hope you can let this go," Comey has testified that Trump told him, referring to the FBI's probe of Flynn.

In written testimony, Comey said that after he and Trump had been talking for a few minutes, Priebus "leaned in through the door by the grandfather clock and I could see a group of people waiting behind him."

"The President waved at him to close the door, saying he would be done shortly. The door closed," Comey wrote.

When he appeared at a public hearing before the Senate Intelligence Committee, Comey said he considered it a "significant fact" that Trump had sought to have the conversation one-on-one, suggesting that the meeting was intended to interfere with the investigation of Flynn.

"Why would you kick the attorney general, the president, the chief of staff out to talk to me if it was about something else?" he asked.

While Trump has repeatedly blasted Mueller's probe, White House lawyers have been eager to show they are cooperating with the special counsel's requests.