Images being shared on social media have sparked fears some pro-Trump rioters at the US Capitol on Wednesday planned on potentially taking hostages.
At least two insurrectionists inside the Capitol building were pictured carrying large zip-ties, which are often used to handcuff people.
A House Democratic aide appeared to confirm zip-ties were spotted on the violent protesters, though it was not clear if he had seen the same photos.
The aide, speaking on condition of anonymity, said an investigation by lawmakers would focus on what intelligence was available in the run-up to the riot, how decisions were made and why the perimeter of the Capitol was not more secure.
It comes as more than 200 members of Congress have demanded Donald Trump be removed from office early following the deadly riot.
The lawmakers, almost all Democrats, either support impeaching the president for a second time or have called on Mr Trump’s cabinet to remove him under the 25th Amendment, NBC News reported.