Former US President Donald Trump had written a speech declaring his victory over his opponent, incumbent President Joe Biden, regardless of the outcome of the election, lawmakers investigating the Republican's involvement in the Capitol riots in January 2021 have found on Thursday.

The findings offer the members of Congress, known as the January 6 panel, evidence about Trump's involvement in the riots many label as an insurrection.

Trump had carefully put together a "premeditated plan" months in advance to claim that he was the righteous winner of the vote regardless of what the final tally showed, panel member Zoe Lofgren told the Congressional hearing.

"The evidence shows that his false victory speech was planned well in advance before any votes had been counted," Lofgren said. She also cited evidence the committee had previously gathered, including testimonies from Trump's one-time campaign manager.

The Jan 6 panel has among its members several who had called for US Attorney General Merrick Garland to charge Trump over the attacks that took place as Congress was trying to ratify Biden's election victory.

The investigation into the riots that took place on January 6 has so far seen more than 700 individuals from nearly all 50 US states charged with crimes related to breaching the Capitol, along with top Trump aides who were implicated.

The attack left seven people dead and more than 100 officers injured, including four officers who later committed suicide.

The committee did not publicly declare that it was going to make criminal referrals, though several US media outlets reported earlier in the day that panel members planned on voting to subpoena Trump during the hearing.

"The vast weight of evidence presented so far has shown us that the central cause of January 6 was one man - Donald Trump - whom many others followed," said committee deputy chair Liz Cheney.

The January 6 panel also underlined its declared position that Trump represents a "clear and present" threat to democracy due to his actions.

"Why would Americans assume that our constitution and our institutions and our Republic are invulnerable to another attack?" Cheney asked.

"A key lesson of this investigation is this our institutions only hold when men and women of good faith make them hold, regardless of the political cost."