Former attorney general Bill Barr said claims of election fraud "precipitated" riots at the US Capitol while comparing the mob to Islamic terrorists.
Asked by ITV News if the debate about the integrity of the election was the final straw, Mr Barr said "that was the thing that precipitated riots on the Hill".
But he stopped short of saying the president's actions constituted incitement to insurrection, as alleged in the articles of impeachment passed by the House.
"I'll leave it to the people who are looking into the genesis of this to say whether incitement was involved," Mr Barr said.
It is the latest break from the president's former ally who resigned on 23 December after publicly saying his Justice Department didn't find election fraud on a scale large enough to influence the outcome of the election.
He first spoke out against Mr Trump in a statement to the Associated Press last week saying the president's conduct was a "betrayal of his office and supporters", and that "orchestrating a mob to pressure Congress is inexcusable".
Mr Trump has continued to make unproven claims the election was stolen at the "Save America Rally" in Washington DC before the riots on 6 January.
"I think it's always important to remember most people are exercising their First Amendment rights, but there is a substantial group obviously that went far beyond that and broke into the Capitol and tried to interfere with the proceedings and that's unacceptable," Mr Barr said.
"I'm sad to see but not surprised in a way to see the kind of violence we saw on Capitol Hill. I don't know if I would use the word inevitable but I think that when you start suppressing free speech, when people lose confidence in the media and also when they lose faith in the integrity of elections, you are going to have some people resort to violence," he added.
Mr Barr said the emergence of political violence from both sides of the ideological spectrum, whether Antifa on the left or Proud Boys on the right, was one of his concerns during his time in the Trump administration.
Asked by if there were similarities between rioters and Islamist terrorist, Mr Barr agreed that many of the people involved in both have "psychological problems or problems with their socialisation", while others just wanted to spread anarchy.
"So to that extent, the raw material of extremism may be similar," Mr Barr said.