Revelations of a roughly eight-hour gap in official records of then-President Donald Trump's phone calls on the day of last year's insurrection at the US Capitol are raising fresh questions about the diligence — or lack thereof — of his record-keeping.

The committee investigating the January 6, 2021 riot has identified a gap in records that stretches a little after 11am to about 7pm that day and involves White House calls, according to two people familiar with the matter.

Trump has not commented but attention surrounding the gap comes alongside a separate potential legal and political headache for the Republican ex-president — the recovery earlier this year of 15 boxes, including records containing classified information from Trump's White House tenure, from his Mar-a-Lago vacation home in Florida.

The 1978 Presidential Records Act requires the preservation of White House documents as property of the US government.

The law was passed in the aftermath of the Watergate scandal, when a collection of secret tapes that President Richard Nixon had considered destroying played a defining role. The tapes revealed that Nixon tried to cover up the bungled burglary of Democratic National Committee headquarters. He chose to resign rather than face impeachment and removal from office.

In theory, the law would require the preservation of emails, text messages and phone records — no matter the device used for the communication, said presidential historian Lindsay Chervinsky.

The problem is, there's no real mechanism to enforce the law, and there's also never been a case where a former commander in chief has been punished for violating the Presidential Records Act. The statute by definition depends on the goodwill of presidents and their staff to police their own record keeping.

"It does require a certain element of good faith and sort of an honour system, and when that crumbles, you can see the limitations," Chervinsky said.

Even so, the January 6 committee is investigating the gap as it works to piece together Trump's communications before and during the insurrection when pro-Trump rioters stormed the US Capitol to prevent Congress from certifying Democrat Joe Biden's 2020 presidential election victory.

House investigators are looking at whether Trump was communicating during that time through other means, possibly through personal cellphones or some other type of communication — such as a phone passed to him by an aide.

Trump has faced attention before over his record-keeping after it was revealed he took boxes of classified materials with him to Mar-a-Lago. House lawmakers opened an investigation and the National Archives and Records Administration revealed that it had asked the Justice Department to look into the matter.

Asked about the issue, Attorney General Merrick Garland has said only that the Justice Department will do what it always does — evaluate the facts and the law "and take it from there".

- AP