Video report by ITV News Security Editor Rohit Kachroo
Former US attorney general William Barr has told ITV News that questioning the legitimacy of the presidential election result "precipitated the riots" in the US Capitol on January 6.
In his first interview since the violent siege of America's seat of political power, Mr Barr said he was "sad" but "not surprised to see the kind of violence we saw".
He described the scenes in the US Capitol earlier this month as "despicable".
Speaking to ITV News Global Security Editor Rohit Kachroo, ahead of Joe Biden's inauguration, Mr Barr said: "Regardless of which side of the political spectrum is involved, we just cannot tolerate violence interfering with the processes of government".
The former attorney general left office in December, after serving under the Trump administration for more than a year.
In an exclusive interview with ITV News, Mr Barr said he would not describe the violence by Trump supporters on January 6 as "inevitable" but continued: "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."
Having served in the role of attorney general twice - before President Trump, under President George W Bush - Mr Barr said this time around he swore in his confirmation hearings that the "emergence of violence in the political process" was one of his concerns.
"It was just starting - people attacking, people wearing Trump hats, and things like that. You had the Proud Boys who were on the right, fighting the Antifa who were on the left," Mr Barr told ITV News.
"I said that I was very worried about the emergence of violence and he had to have zero tolerance for it."
Quizzed on the similarities between the rioters and the threat of Islamist terrorists, Mr Barr said there are "commonalities" between the two groups.
"There is obviously some commonality because many of the people that get involved in this are people who have problems - psychological problems or problems with their socialisation.
"So to that extent, the raw material of extremism may be similar."
The former attorney general said some groups were just "anarchists against any kind of order".
"The Islamist threat is far more programmatic," Mr Barr said.
"There's probably more structure to it than what we have to deal with here".