Testimony from former Attorney General Bill Barr was heard by a congressional panel on Monday.
It comes amid insight into fractures within the Trump camp over his claims of election fraud.
During Monday’s session it was heard that two camps emerged within Mr Trump’s campaign team: those who called themselves “Team Normal” and accepted Mr Trump lost the election to Joe Biden, and those who did not.
The committee is investigating the former US President’s role in the insurrection.
Ahead of Monday’s session, it was announced that Mr Trump’s former campaign manager, Bill Stepien, would not be giving testimony because his wife had gone into labour.
Instead, his lawyer gave a statement on his behalf.
Mr Stepien’s previous private testimony was also publicly played by the Democratic-led US House of Representatives select committee.
In it, he revealed that people within Mr Trump’s inner circle urged him not to declare victory following the election.
Those within “Team Normal” told Mr Trump that he had lost the election, Mr Stepien said, while others refuted the result.
This included ex-New York Mayor Rudy Giuliani, who was vocal in his support of Mr Trump’s claim the election had been stolen.
Among those who urged the former president not to declare victory was Bill Barr, who said he had told Mr Trump there was no proof to back his claims of rigged voting machines or ballot “dumps”.
Mr Barr said the claims were “crazy stuff”.
However, Mr Trump refused to listen to his concerns and continued claiming the election was fraudulent.
Mr Barr said in video testimony that he was left “demoralised” by it.
“I thought, ‘Boy if he really believes this stuff, he has lost contact with — he’s become detached from reality, if he really believes this stuff,’” he said.
The 6 January select committee is seeking to establish Mr Trump’s election fraud claims directly led to an attack on the US Capitol, which sent shockwaves around the world.
Four people died the day of the attack, one fatally shot by police and the others of natural causes.
Some 140 police officers were injured, and one died the next day. Four officers later died by suicide.
Nearly 850 people have been arrested for crimes related to the riot, including more than 250 charged with assaulting or impeding law enforcement.