US President Joe Biden has apologised after some members of the National Guard stationed at the Capitol were pictured sleeping in a car park.
More than 25,000 troops were deployed to Washington DC for his inauguration after violence earlier this month.
Images spread on Thursday showing them forced to rest in a nearby parking garage after lawmakers returned.
The conditions sparked anger among politicians, and some state governors recalled troops over the controversy.
Mr Biden called the chief of the National Guard Bureau on Friday to apologise and ask what could be done, according to US media reports.
First Lady Jill Biden also visited some of the troops to thank them personally, bringing biscuits from the White House as a gift.
"I just wanted to come today to say thank you to all of you for keeping me and my family safe," she said.
The photographs showing hundreds of troops in a parking garage went viral on Thursday and sparked outrage, including from members of Congress.
Many voiced concerns about the conditions, with guardsmen exposed to car fumes and without proper access to facilities like toilets after having been on alert for days.
Images of the cramped conditions also sparked fears about the spread of coronavirus.
A US official, speaking anonymously to Reuters news agency, said on Friday that between 100 and 200 of those deployed had tested positive for Covid-19. The figure - which would represent a small proportion of the more than 25,000 deployed, has not been publicly confirmed.
Chuck Schumer, a Democrat and the new Senate majority leader, said that the move was "an outrage" and pledged it "will never happen again".
Ron DeSantis, Florida's governor, was among those who said he had ordered guards from his state to return home following the controversy.
"This is a half-cocked mission at this point and the appropriate thing is to bring them home," he told Fox News on Friday.
The Senate Rules Committee is also investigating the issue, Senator Roy Blunt told Politico.
There are conflicting reports about why the troops were moved from the Capitol.
A National Guard spokesman told US media they were moved on Thursday afternoon at the request of the Capitol Police because of "increased foot traffic" as Congress came back into session.
The acting chief of the Capitol Police, Yogananda Pittman, later said her agency "did not instruct the National Guard to vacate the Capitol Building facilities", while two officers contradicted her statement in comments to the Associated Press news agency.
The decision was reversed later on Thursday when the troops were allowed to return to the Capitol.
A joint statement from the US National Guard and US Capitol Police on Friday said they had worked together to make sure those in the Capitol Complex had "appropriate spaces" to take on-duty breaks.
They also said off-duty troops were being housed in hotel rooms or other accommodation and thanked members of Congress for their concern.
