When asked if CMS was warned about the system's problems ahead of the launch, Slavitt answered yes.
That was enough for GOP Rep. Renee Ellmers of North Carolina to say: "The bottom line here is that CMS is responsible for this failure."
At the White House, spokesman Jay Carney told reporters on Thursday that administration officials "did not anticipate, we did not know of the kind of problems that would take place beginning on Oct. 1 until Oct. 1 came and we saw these challenges."
He repeated the president's insistence that the problems will be fixed, an assurance also provided by Campbell to the House panel.
"The system is working, people are enrolling, but people will be able to enroll at a faster pace" as troubles get resolved, she told the panel, adding that her company expects the online system to be able to enroll Americans seeking health insurance by Jan. 1.
She rejected media reports that the system needs to be scrapped or that five million lines of code must be rewritten.
Campbell also said the software program was unprecedented, noting that it "for the first time in history" combines the process of enrolling and selecting health insurance with determining eligibility for government subsidies "all in one place."
In her written testimony, Campbell said that "unfortunately, in systems this complex with so many concurrent users, it is not unusual to discover problems that need to be addressed once the software goes into a live production environment."
"This is true regardless of the level of formal end-to-end performance testing -- no amount of testing within reasonable time limits can adequately replicate a live environment of this nature," she added.
To accentuate progress on the website, two Democratic panel members said during the hearing that either they or their offices were able to use the website to determine health insurance options available to them on Wednesday or Thursday.
However, Republicans said they had failed to gain full access to the website despite repeated attempts and complained they hear similar stories from their constituents across the country.
The White House and Health and Human Services Secretary Kathleen Sebelius say the initial problems in accessing the website have eased.
Sebelius has brought in tech experts from Silicon Valley and elsewhere to work with the contractors on eliminating problems, with former acting Office of Management and Budget Director Jeff Zients overseeing the effort.
"The work of constantly improving the website will continue day by day," Carney said Wednesday. "A week from now, it's going to be better than it is today."
The Energy and Commerce Committee isn't the only one probing the ACA in light of its issues. So, too, is the House Ways and Means Committee and the House Oversight and Government Reform Committee.
The latter sent an eight-page letter Wednesday to 11 contracting firms involved in putting together HealthCare.gov giving them two days to provide documents, meeting lists and all communications with the Obama administration on the project, committee spokeswoman Caitlin Carroll said.
The fact all three committees are in the same chamber -- which is led by Republicans -- is likely no coincidence. The GOP has and continues to staunchly oppose the health care law, even trying unsuccessfully to make defunding or delaying it requirement before it would fund the government.
The White House has pushed back against Republicans and some Democrats who have urged an extension of the open enrollment period beyond March 31 and/or a delay in penalties for those who don't sign up for insurance on time because of the website issues.
The Obama administration is moving to clarify confusion on when people need to sign up for health care, and it has plans to issue new guidelines to this point soon.
Still, Health and Human Services spokeswoman Joanne Peters insisted "the individual mandate timing" -- i.e. the requirement for a person to have insurance by March -- "has not changed. ... It was true this morning. It is true tonight."