FOUR former advisers and associates of Donald Trump, including Steve Bannon, have been subpoenaed by a committee investigating the Capitol insurrection.

The House committee looking into events on January 6 when Trump was still president have also summoned former White House Chief of Staff Mark Meadows, former White House Deputy Chief of Staff for Communications Dan Scavino, and former Defence Department official Kashyap Patel.

Committee chairman Bennie Thompson wrote to the four men to say it is investigating "the facts, circumstances, and causes" of the attack and asked them to produce documents and appear at depositions in mid-October.

The subpoenas are a significant escalation for the panel, which is now launching the interview phase of the investigation after sorting through thousands of pages of documents the committee requested from federal agencies and social media companies.

The goal is to provide a complete account of what went wrong when the Trump loyalists quickly overwhelmed police and interrupted the certification of President Joe Biden's victory – and to prevent anything like it from ever happening again.

Thompson said in letters to each of the witnesses that investigators believe they have relevant information about the lead-up to the insurrection.

In the case of Bannon, for instance, Democrats cite his prediction on January 5 that "hell is going to break loose tomorrow" and his communications with Trump one week before the riot in which he urged the president to focus his attention on January 6.

The committee also cites Meadows' work to overturn Trump's defeat in the weeks prior to the insurrection and his pressure on state officials to push the former president's false claims of widespread voter fraud.

In the letter to Meadows, Democrats say they have "credible evidence" of his involvement in events within the scope of the committee's investigation.

That includes his communication with Trump on January 6 and his reported involvement in the "planning and preparation of efforts to contest the presidential election and delay the counting of electoral votes".

Thompson also signalled that the committee is interested in Meadows' requests to Justice Department officials for investigations into potential election fraud.

Former attorney general William Barr has said the Justice Department did not find fraud that could have affected the election's outcome.

The panel cites reports that Patel, a Trump loyalist who had recently been placed at the Pentagon, was talking to Meadows "non-stop" the day the attack unfolded.

Scavino was with Trump on January 5 during a discussion about how to persuade members of Congress not to certify the election for Joe Biden, according to reports cited by the committee.

On Twitter, he promoted Trump's rally ahead of the attack and encouraged supporters to "be a part of history".

The panel said its records indicate that Scavino was "tweeting messages from the White House" on January 6.