The House select committee investigating the January 6 Capitol attack is closely focused on phone calls and conversations among Donald Trump’s children and top aides captured by a documentary film-maker weeks before the 2020 election, say sources familiar with the matter.

The calls among Trump’s children and top aides took place at an invitation-only event at the Trump International hotel in Washington DC that took place the night of the first presidential debate on 29 September 2020, the sources said.

The select committee is interested in the calls, the sources said, since the footage is understood to show the former president’s children, including Donald Jr and Eric Trump, privately discussing strategies about the election at a crucial time in the presidential campaign.

House investigators first learned about the event, hosted by the Trump campaign, and the existence of the footage through British film-maker Alex Holder, who testified about what he and his crew recorded during a two-hour interview last week, the sources said.

The film-maker testified that he had recorded around seven hours of one-to-one interviews with Trump, then-vice president Mike Pence, Trump’s adult children and Trump’s son-in-law Jared Kushner, the sources said, as well as around 110 hours of footage from the campaign.

But one part of Holder’s testimony that particularly piqued the interest of the members of the select committee and chief investigative counsel Tim Heaphy was when he disclosed that he had managed to record discussions at the 29 September 2020 event.

The select committee is closely focused on the footage of the event – in addition to the content of the one-on-one interviews with Trump and Ivanka – because the discussions about strategies mirror similar conversations at that time by top Trump advisors.

On the night of the first presidential debate, Trump’s top former strategist Steve Bannon said in an interview with HBO’s The Circus that the outcome of the 2020 election would be decided at the state level and eventually at the congressional certification on January 6.

“They’re going to try and overturn this election with uncertified votes,” Bannon said. Asked how he expects the election to end, Bannon said: “Right before noon on the 20th, in a vote in the House, Trump will win the presidency.”

The select committee believes that ideas such as Bannon’s were communicated to advisors to Donald Jr and his fiancee, Kimberly Guilfoyle, even before the 2020 election had taken place, the sources said – leading House investigators to want to review the Trump hotel footage.

What appears to interest the panel is whether Trump and his children had planned to somehow stop the certification of the election on January 6 – a potential violation of federal law – and to force a contingent election if Trump lost as early as September.

The event was not open to the public, Holder is said to have testified, and the documentary film-maker was waved into the Trump hotel by Eric Trump. At some point after Holder caught the calls on tape, he is said to have been asked to leave by Donald Jr.

Among the conversations captured on film was Eric Trump on the phone to an unidentified person saying, according to one source familiar: “Hopefully you’re voting in Florida as opposed to the other state you’ve mentioned.”

The phone call – a clip of which was reviewed by the Guardian – was one of several by some of the people closest to Trump that Holder memorialized in his film, titled Unprecedented, which is due to be released in a three-part series later this year on Discovery+.

Holder also testified to the select committee, the sources said, about the content of the interviews. Holder interviewed Trump in early December 2020 at the White House, and then twice a few months after the Capitol attack both at Mar-a-Lago and his Bedminster golf club.

The select committee found Holder’s testimony and material more explosive than they had expected, the sources said. Holder, for instance, showed the panel a discrepancy between Ivanka Trump’s testimony to the panel and Holder’s camera.

In her interview in December 2020, the New York Times earlier reported, Ivanka said her father should “continue to fight until every legal remedy is exhausted” because people were questioning “the sanctity of our elections”.

That interview was recorded nine days after former attorney general William Barr told Trump there was no evidence of election fraud. But in her interview with the select committee, Ivanka said she had “accepted” what Barr had said.