• Pat Cipollone, Trump's former White House counsel, met with the Jan. 6 select committee behind closed doors for a transcribed interview.
• Cipollone was on Capitol Hill for eight and a half hours.
• The committee is gathering evidence on former President Donald Trump's involvement in the Capitol riot.
• Ex-White House aide Cassidy Hutchinson and other witnesses referenced Cipollone repeatedly in sworn public testimony before the committee.

The House select committee investigating the Jan. 6 Capitol riot interviewed one of its most sought-after witnesses on Friday as it continues to gather evidence implicating former President Donald Trump in the attack.

Pat Cipollone, Trump's former White House counsel, met with the committee behind closed doors for a transcribed interview that lasted eight and a half hours. He spent more than an hour of that time outside the deposition room conferring with his lawyers, and departed the Capitol Hill office building after 5 p.m. ET. He did not answer reporters' questions.

Cipollone was subpoenaed last week by the panel, which said it has obtained evidence that he "repeatedly raised legal and other concerns about President Trump's activities on January 6th and in the days that preceded."

Cipollone, who spoke with investigators informally in April, has taken center stage in some of the committee's recent public hearings through the testimony of other witnesses who cited his numerous warnings to Trump.

The panel is hoping he could confirm some of the most incendiary allegations shared last week by Cassidy Hutchinson, a former top aide to Trump's ex-chief of staff Mark Meadows.

Ahead of the Jan. 6 riot, Cipollone warned staff against allowing Trump to meet protestors at the U.S. Capitol where lawmakers were meeting to confirm President Joe Biden's electoral victory, according to Hutchinson's testimony. "We're going to get charged with every crime imaginable" if Trump followed through on those plans, she recalled him saying.

"It would look like we were obstructing justice," Cipollone explained, according to Hutchinson.

Trump told his followers he would meet them at the Capitol at a rally near the White House that shortly preceded the riot. But his security detail refused to take him to the Capitol and he ultimately returned to the White House, at which point Cipollone urged Meadows to act to get Trump to quell the attack, Hutchinson testified.

"I remember Pat saying something to the effect of, 'Mark, we need to do something more. They're literally calling for the vice president to be effing hung,'" she said.

"Mark had responded something to the effect of, 'You heard him, Pat. He thinks Mike [Pence] deserves it. He doesn't think they're doing anything wrong,'" she said.

Cipollone did not respond to CNBC's request for comment. A spokesman for the select committee declined to comment.

The interview was set to take place just four days before the committee's next public hearing, when it is to lay out the initial findings from its yearlong investigation into the Jan. 6, 2021, riot at the U.S Capitol by a violent mob of Trump's supporters.

The committee has yet to reveal the topic of its upcoming hearing, scheduled for 10 a.m. ET on Tuesday. Chair Bennie Thompson, D-Miss., said last month that the panel would hold its final two public hearings in July. Vice Chair Liz Cheney, R-Wyo., previously said that those hearings will show how Trump illegally directed his supporters toward the Capitol, and then failed to take quick action to stop the riot once it began.

But the committee has changed its plans to accommodate new evidence — most notably, by adding last week's public hearing last week to highlight Hutchinson's bombshell testimony.

Before being subpoenaed, Cipollone had resisted the committee's efforts to bring him in for an interview. Cheney openly appealed to Cipollone to cooperate, saying in a prior hearing that Trump "does not want Mr. Cipollone to testify here" because the evidence shows that he and his office "tried to do what was right."

Trump, in a post on his social media platform Thursday morning, suggested the committee's push to speak with Cipollone would make future presidents less willing to discuss sensitive issues with their White House counsel. "So bad for USA!" Trump wrote on Truth Social.