The House January 6 committee has voted to subpoena Donald Trump to provide testimony before the panel over the 2021 attack at the Capitol.

The panel voted unanimously to compel the former president to appear.

"We must seek the testimony under oath of January 6th's central player," said Rep. Liz Cheney, R-Wyo., the committee's vice-chair.

The vote seeking Trump's testimony comes as the panel is producing vivid new details and evidence of Trump's state of mind as he refused to concede his election loss to Joe Biden, resulting in the 2021 attack at the Capitol.

The January 6 panel is showing previously unseen footage of congressional leaders phoning officials for help during the Capitol siege.

House Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer can be seen talking to governors in neighbouring Virginia and Maryland. Later the footage shows Senate Republican Leader Mitch McConnell and other GOP leaders as the group asks the acting attorney general for help.

"They're breaking the law in many different ways — quite frankly at the instigation of the president of the United States," Pelosi is heard saying at one point.

In never-before-seen Secret Service messages, the panel produced evidence of the way extremist groups provided the muscle in the fight for Trump's presidency, planning weeks before the attack to send a violent force to Washington.

"Their plan is literally to kill people," read a tip that was sent to Secret Service more than a week before the violence on January 6.

The Secret Service warned in a December 26, 2020, email of a tip that members of the right-wing Proud Boys planned to march in Washington on January 6 with a group large enough to outnumber the police.

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"It felt like the calm before the storm," one Secret Service agent wrote in a group chat.

The House panel warned that the insurrection at the Capitol was not an isolated incident but a warning of the fragility of the nation's democracy in the post-Trump era.

"None of this is normal or acceptable or lawful in a republic," Republican Rep Liz Cheney said.

"There is no defence that Donald Trump was duped or irrational. No president can defy the rule of law and act this way in a constitutional republic, period."

The 10th public session, just weeks before the congressional midterm elections, was delving into Trump's "state of mind," said Democratic Chairman Bennie Thompson.

The committee is starting to sum up its findings that Republican Trump, after losing the 2020 presidential election, launched an unprecedented attempt to stop Congress from certifying Democrat Biden's victory. The result was the mob storming of the Capitol.

Statements from Thompson and Cheney were laden with language frequently seen in criminal indictments. Both lawmakers described Trump as "substantially" involved in the events of Jan 6 Cheney said Trump had acted in a "premeditated" way.

To illustrate what it said were "purposeful lies," the committee juxtaposed repeated instances in which top administration officials recounted telling Trump the actual facts with clips of him repeating the exact opposite at his pre-riot rally at the Ellipse on Jan. 6.

The committee may well decide on whether to make a criminal referral to the Justice Department.

Vice Chair Liz Cheney, R-Wyo., speaks as the House select committee investigating the Jan 6 attack on the US Capitol. Photo / AP

Thursday's hearing opened at a mostly empty Capitol complex, with most lawmakers at home campaigning for reelection. Several people who were among the thousands around the Capitol on Jan 6 are now running for congressional office, some with Trump's backing. Police officers who fought the mob filled the hearing room's front row.

To describe the president's mindset, the committee divulged new material, including interviews with Trump's top Cabinet officials, aides and associates in which some described the president acknowledging that he had lost.

In one, according to ex-White House official Alyssa Farah Griffin, Trump looked up at a television and said, "Can you believe I lost to this (expletive) guy?"

The committee is also drawing on the trove of 1.5 million documents it received from the US Secret Service, including an email from Dec 11, 2020, the day the Supreme Court rejected one of the main lawsuits Trump's team had brought against the election results.

"Just FYI. POTUS is pissed," the Secret Service wrote, according to documents obtained by the committee.

White House aide Cassidy Hutchinson, a top aide to then-chief of staff Mark Meadows, recalled Trump as being "livid" and "fired up" about the court's ruling.

Trump told Meadows "something to the effect of: 'I don't want people to know we lost, Mark. This is embarrassing. Figure it out,'"Hutchinson told the panel in a recorded interview.

Cabinet members including former Secretary of State Mike Pompeo, Attorney General William Barr and Labor Secretary Eugene Scalia also said in interviews shown at the hearing that they believed that once the legal avenues had been exhausted, that should have been the end of Trump's efforts to remain in power.

"In my view, that was the end of the matter," Barr said of the Dec 14 Electoral College vote.

But rather than the end of Trump's efforts to stay in power, the committee signalled it was only the beginning — as the president summoned the crowd to Washington for a rally to fight the election on Jan 6.

The session was serving as a closing argument for the panel's two Republican lawmakers, Cheney of Wyoming and Adam Kinzinger of Illinois, who have essentially been shunned by Trump and their party and will not be returning in the new Congress. Cheney lost her primary election, and Kinzinger decided not to run.

Another committee member, Rep Elaine Luria, D-Va., a retired Naval commander, is in a tough reelection bid against state Sen. Jen Kiggans, a former Navy helicopter pilot.

The panel was expected to share information from its recent interviews — including testimony from Ginni Thomas, the conservative activist and wife of Supreme Court Justice Clarence Thomas. She was in contact with the White House during the run-up to Jan 6.

The committee, having conducted more than 1,500 interviews and obtained countless documents, has produced a sweeping probe of Trump's activities from his defeat in the November election to the Capitol attack.

"He has used this big lie to destabilise our democracy," said Rep. Zoe Lofgren, D-NY, who was a young House staff member during the Richard Nixon impeachment inquiry in 1974. "When did that idea occur to him and what did he know while he was doing that?"

This week's hearing is to be the final presentation from lawmakers before the midterm elections. But staff members say the investigation continues.

The Jan 6 committee has been meeting for more than a year, set up by the House after Republican senators blocked the formation of an outside panel similar to the 9/11 commission set up after the 2001 terrorist attacks. Even after the launch of its high-profile public hearings last summer, the Jan 6 committee continued to gather evidence and interviews.

Under committee rules, the Jan 6 panel is to produce a report of its findings, likely in December. The committee will dissolve 30 days after the publication of that report, and with the new Congress in January.

House Republicans are expected to drop the Jan 6 probe and turn to other investigations if they win control after midterm elections, primarily focusing on Biden, his family and his administration.

At least five people died in the Jan 6 attack and its aftermath, including a Trump supporter shot and killed by Capitol Police.

Police engaged in often bloody, hand-to-hand combat, as Trump's supporters pushed past barricades, stormed the Capitol and roamed the halls, sending lawmakers fleeing for safety and temporarily disrupting the joint session of Congress certifying Biden's election.

More than 850 people have been charged by the Justice Department in the Capitol attack, some receiving lengthy prison sentences for their roles. Several leaders and associates of the extremist Oath Keepers and Proud Boys have been charged with sedition.

Trump faces various state and federal investigations over his actions in the election and its aftermath.

- AP