Top US virus expert Dr Anthony Fauci, a member of the Covid task force, said it was clear there was a "superspreader event" at the White House.
When asked on CBS News radio about the White House outbreak, Dr Fauci said "the data speak for themselves".
Dr Fauci's comments come as President Donald Trump says he will get back on the election trail by speaking to supporters outdoors at the White House later today, then hold a rally in Florida in two days' time.
The 15 October televised presidential debate between Trump and Democratic nominee Joe Biden will not proceed, the Commission on Presidential Debates said in a statement, adding that both campaigns had announced "alternate plans for that date."
The commission would focus on the final presidential debate due on 22 October, a spokesperson said.
The cancellation became a possibility once Trump made it clear he would not accept plans to hold the debate as a virtual event so that others would not be exposed to Covid-19.
Trump was scheduled to do an on-camera interview with Fox News on Friday night (US time), his first since being diagnosed. Fittingly given he is a former reality television star, Fox said in a press release that Dr Marc Siegel "will conduct a medical evaluation and interview during the programme".
Biden has continued to campaign during Trump's illness, with his next events scheduled in Las Vegas, Nevada.
The former vice president, who has sharply criticised Trump's handling of the pandemic, is leading in national polls but the advantage is more narrow in some of the swing states that may determine the election outcome.
A new Reuters/Ipsos poll showed Americans are steadily losing confidence in how Trump has managed the health crisis - with his net approval on the issue hitting a new low.
CBS News Radio White House correspondent Steven Portnoy asked what Dr Fauci, a veteran health official, thought of the White House's reluctance to insist on mask-wearing and social distancing as virus precautions, and instead rely on regular testing.
"The data speak for themselves - we had a superspreader event in the White House, and it was in a situation where people were crowded together and were not wearing masks."
Dr Fauci also noted experts have been recommending mask-wearing for the last six months.
An event at the White House, celebrating the president's nomination of Amy Coney Barrett as a Supreme Court judge on Saturday 26 September, is thought to be the root of the localised outbreak, as many attendees have since tested positive.
Large gatherings are still banned in the nation's capital due to Covid-19, but federal property like the White House is exempt.
'Law and order' theme
A White House official said Trump will address the crowd from a White House balcony on a "law and order" theme. A source familiar with the situation said the crowd could be in the hundreds, and all were expected to wear masks.
Trump on Monday night (US time) will stage his first campaign rally since his coronavirus diagnosis in Sanford in central Florida, a state crucial to his re-election hopes. The event will take place at an airport, and the campaign did not disclose if it would be held in a hangar with its doors open, as it has in the past, or entirely outside.
Questions remain about whether Trump is still contagious.
White House physician Sean Conley said in a memo released yesterday that Trump had completed his course of therapy for Covid-19, he had remained stable since returning home from the Walter Reed medical facility on Tuesday and could resume public engagements today.
"There are medical tests under way that will ensure that when the president is back out he will not be able to transmit the virus," White House spokeswoman Kayleigh McEnany said, adding Conley would lay out the details later. "He won't be out there if he can transmit the virus."
McEnany is one of a string of Trump aides, including his campaign manager, who have tested positive in the last week as the virus spread within the White House and Trump campaign.
Trump tested positive for Covid-19 last Thursday (Friday NZ time), according to his doctors, but it remains unclear when his last negative test was.
As many as 34 White House aides and other contacts have tested positive for Covid-19 in recent days, according to an internal memo viewed by ABC News.
Although the names of many people who have interacted with the president and tested positive are now known, it remains unclear just how many were exposed at the White House.
New Covid safety measures - including more stringent mask wearing - are now in place there.
- BBC/ Reuters