House Speaker Nancy Pelosi on Thursday formally announced that lawmakers will create a select committee to investigate the deadly US Capitol riot, weeks after Senate Republicans blocked formation of a bipartisan, 9/11-style commission.

"This morning, with great solemnity and sadness, I am announcing that the House will be establishing a select committee on the January 6 insurrection," Democrat Pelosi told reporters.

Pelosi described the attack on the citadel of democracy by supporters of then-president Donald Trump as a "day of darkness" for the nation that requires a thorough investigation.

"It is imperative that we establish the truth of that day, and ensure that an attack of that kind does not happen" in the future.

Pelosi also leveled searing criticism at the "cowardly" Republicans who she said obliged when Senate Minority Leader Mitch McConnell asked them to vote against creation of the broader, bipartisan commission one month ago.

Six Republicans broke ranks and voted with Democrats to launch the commission, but it was four short of the number needed to advance the bill. 

In a display of loyalty to Trump -- and likely concerned about how the findings of an investigation might impact the 2022 midterm elections -- the other Republicans voted against launching the probe. 

Pelosi waved off concern that the select committee's findings might be dismissed by the American public because it is not a fully bipartisan commission.

The committee will be headed by a Democrat but the speaker said she hopes the House's top Republican will appoint "responsible people" to it.

Few details of the panel have been formally released but the committee is expected to have subpoena power. 

As for the timing of the committee's work and its final report, Pelosi stressed a comprehensive investigation rather than speedy conclusions.

"The timetable will be as long as it takes," she said.