Donald Trump called a witness who has yet to appear before a congressional committee investigating the 2021 US Capitol attack, committee vice chairperson Liz Cheney has said, raising concerns the former president might be illegally trying to influence witness testimony.

Speaking at the end of a three-hour hearing Ms Cheney, one of two members from Trump's Republican Party on the nine-member panel, announced that it had referred the matter to the US Justice Department.

The witness did not pick up Trump's call but did tell their lawyer, said Ms Cheney, vice chair of the House of Representatives 6 January committee.

"Their lawyer alerted us. And this committee has supplied that information to the Department of Justice," Ms Cheney said in her closing remarks. "Let me say one more time: We will take any effort to influence witness testimony very seriously."

The call occurred after the committee's last hearing on 28 June.

Liz Cheney during today's hearing

A Justice Department spokesman declined to comment on whether Mr Trump may have tried to intimidate a witness. A representative for the former president did not immediately respond to a request for comment.

Ms Cheney said in a television interview broadcast last Thursday that the panel may make a criminal referral to the Justice Department recommending that anybody who tried to influence testimony be prosecuted.

The witness-tampering issue emerged at the committee's hearing a week ago, when Ms Cheney disclosed that some witnesses reported receiving veiled threats from Trump allies to do "the right thing."

Democrat Jamie Raskin, another committee member, told CNN after the hearing: "Witness tampering is a crime in the District of Columbia. It's a crime in federal law, and it's a form of obstruction of justice."

"We want it to stop ... I'll leave it to the Department of Justice to decide where to take it from there".

US politicians have accused Mr Trump of inciting a mob of followers to attack the US Capitol on 6 January 2021, in a last-ditch bid to remain in power as Congress was formally certifying the then-president's election loss.

In video testimony shown by the House of Representatives committee investigating the riot, witnesses described a loud late-night six-hour meeting on 18 December 2020, where Mr Trump disregarded White House staffers who urged him to concede the November 2020 election to Democrat Joe Biden.

Instead, Mr Trump sided with outside advisers who urged him to keep pressing his baseless claims of election fraud. Committee members said Mr Trump ultimately was responsible for the chaos that followed.

"President Trump is a 76-year-old man. He is not an impressionable child ... He is responsible for his own actions and his own choices," said Ms Cheney.

Committee members said Mr Trump incited the riot through his refusal to admit he lost the election and through comments like his 19 December 2020 Twitter post, shortly after the six-hour meeting, for supporters to flock to Washington for a "big protest," saying, "Be there, will be wild."

The panel's seven Democrats and two Republicans have used the hearings to build a case that Mr Trump's efforts to overturn his defeat in the 2020 election constitute illegal conduct, far beyond normal politics.

Mr Trump, a Republican who has hinted he will seek the White House again in 2024, denies wrongdoing and has falsely asserted that he lost only because of widespread fraud that benefited Mr Biden, a Democrat.

The committee played recorded testimony from Trump administration figures describing the angry meeting on 18 December where a handful of Mr Trump's outside advisers, including his personal lawyer Rudy Giuliani, attorney Sidney Powell and Patrick Byrne, chief executive of Overstock.com, encouraged him to fight the election result.

A video deposition by former White House Counsel Pat Cipollone is shown during the hearing

"I don't think any of these people were providing the president with good advice. I didn't understand how they had gotten in," said Pat Cipollone, the former White House counsel.

Mr Raskin displayed a text from White House aide Cassidy Hutchinson, who gave explosive testimony last month, saying of the meeting, "The West Wing is unhinged."

The meeting lasted more than six hours, ending after midnight.

Mr Giuliani, who was escorted out of the White House grounds, said in video testimony his argument had been: "You guys are not tough enough. Or maybe I put it another way: You're a bunch of pussies, excuse the expression. I'm almost certain the word was used."

The attack on the Capitol, following a speech Mr Trump gave at a rally outside the White House, delayed certification of Joe Biden's election for hours, injured more than 140 police officers and led to several deaths.

The committee presented evidence that it said showed Mr Trump's call for his supporters to march on the Capitol was not spontaneous but had been planned in advance.

The panel showed an unsent Twitter message about the rally, with a stamp showing Mr Trump had seen it: "Please arrive early, massive crowds expected. March to the Capitol after. Stop the Steal!"

The committee also played audio testimony from a former employee of Twitter describing his fear after Mr Trump's December tweet and deep concern on 5 January about the possibility of violence on 6 January.

"It felt as if a mob was being organised and they were gathering together their weaponry and their logic and their reasoning behind why they were prepared to fight," the Twitter employee said, his voice disguised.

Panel members also showed how White House aides became increasingly concerned about the plans.

"Things have gotten crazy and I desperately need some direction," White House aide Katrina Pierson wrote to White House Chief of Staff Mark Meadows on 2 January.

About 800 people, including members of both right-leaning groups, have been charged with taking part in the Capitol riot, with about 250 guilty pleas so far.

Former Oath Keepers spokesman Jason Van Tatenhove, right, and Stephen Ayres

The hearing also looked at links between right-wing militant groups, including the Oath Keepers, Proud Boys and the QAnon internet conspiracy movement, with Mr Trump and his allies. Many Oath Keepers and Proud Boys participated in the attack.

Two witnesses testified in the hearing room - Stephen Ayres, who has pleaded guilty to a federal charge after participating in the attack on the Capitol, and Jason Van Tatenhove, a former spokesperson for the Oath Keepers.

"In my opinion, the Oath Keepers are a very dangerous organisation," Van Tatenhove said.

Mr Trump and his supporters - including many Republicans in Congress - dismiss the 6 January panel as a political witch hunt, but the panel's backers say it is a necessary probe into a violent threat against democracy.