The unprecedented January 6 attack on the seat of Congress left five dead and led the House to impeach Trump a second time, for a fiery speech that day in which he urged thousands of his followers to fight Democratic President-elect Joe Biden's victory.

The hundreds who stormed the Capitol quickly overwhelmed police guarding the building, calling security preparations into question, and showed a level of detailed knowledge about the location of lawmakers' officers by some rioters that raised suspicions.

Democratic Representative Mikie Sherrill, a former US Navy helicopter pilot, has accused some Republican lawmakers of helping Trump supporters, saying she saw colleagues leading groups on "reconnaissance" tours on January 5.

Pelosi was asked at a news conference whether any actions would be taken against lawmakers who may have brought tours to the Capitol the day before the attack, which briefly halted Congress in its duty of certifying Biden's win.

"If in fact it is found that members of Congress were accomplices to this insurrection, if they aided and abetted the crimes, there have to be actions taken beyond the Congress, in terms of prosecution for that," Pelosi said.

More than 30 House Democrats, including Sherrill, have asked the acting House and Senate serjeants-at-arms and the acting head of the Capitol Police for information about who was in the building on January 5.

Pelosi called for lawmakers to review who organised the siege by Trump followers, some of whom were aligned with white supremacist groups. The security review would look into the Capitol's security infrastructure, procedures, and "command and control", she said.

The march by Trump's supporters had been publicised for weeks.

The US Justice Department's internal watchdog will review how the FBI and other law enforcement agencies prepared and responded, Inspector-General Michael Horowitz said on Friday.

The House on Wednesday approved an article of impeachment charging Trump with incitement of insurrection for his fiery speech immediately before the attack. Pelosi did not answer questions about when she would send the article to the Senate, the step necessary to trigger trial.

"You'll be the first to know when we announce that we're going over there," Pelosi told reporters.

Trump is not expected to face trial before his term ends on January 20, though lawmakers could bar him from running for office again.

Congressional Democrats have been grappling with how to balance the impeachment of Trump with other Senate priorities including confirming Cabinet nominees and addressing Biden's request for a $US1.9 trillion ($A2.5 trillion) coronavirus relief package.

When Biden takes office on Wednesday, he will lead a nation where more than 388,000 people have died of COVID-19, which is infecting more than 235,000 people a day.

"This is a matter of complete urgency," Pelosi said of the COVID-19 vaccine distribution under Biden's proposal.

According to Senate rules, the trial would start the day after the charge is sent over by the House. The last time the House voted to impeach Trump, in December 2019, Pelosi did not transmit the charges to the Senate for weeks.

Those charges related to abuse of power and obstruction of Congress stemming from Trump's request that Ukraine investigate Biden and his son Hunter. The Senate in February 2020 voted to acquit Trump.

The Senate is scheduled to return to Washington on Tuesday, the day before Biden's inauguration.

A inauguration traditionally draws hundreds of thousands of visitors but the ceremonies have been scaled back dramatically because of the COVID-19 pandemic, even before last week's riot.

Now Washington is wrapped in a security blanket with thousands of troops and law enforcement officers on duty.

Meanwhile, Trump plans to leave Washington on the morning of the inauguration and go to his club in Palm Beach, Florida, sources say.

Australian Associated Press