Some of the criticism of the website's launch has to do with what Obama and other officials knew and when they knew it.
CNN has learned the administration received stark warnings just one month before the launch that the federal health care site was not ready to go live, according to a confidential report.
The caution, from the main contractor CGI Federal, warned of a number of risks and issues for HealthCare.gov, even as company executives were testifying publicly that the project had achieved key milestones.
Sebelius told the committee that the outside contractors that built the website never recommended delaying the Oct. 1 launch.
She admitted that "we did not adequately do end-to-end testing" of the website and that various components "were not locked and loaded into the system" until mid-September.
The contracts with the private companies working on the health care website do not have any "built-in penalties" allowing her department to charge them for disappointing or faulty work, Sebelius said. However, she said the agency will not pay for incomplete work.
Republican Rep. Mike Rogers of Michigan, who chairs the House Intelligence Committee, accused Sebelius of putting the private information of Americans at risk by failing to properly test security measures on the website.
"This is a completely unacceptable level of security," he said. "You know it's not secure."
Sebelius responded that testing occurs regularly, and she told Rogers she would get back to him on whether any end-to-end security test of the entire system has ever occurred. Rogers responded that he knows there have been no such comprehensive security tests.
An internal government memo, obtained by CNN on Wednesday and written just days before the website opened, warned of a "high" security risk because of a lack of testing.
"Due to system readiness issues, the (security control assessment) was only partly completed," said the memo from the Centers for Medicare and Medicaid Services. "This constitutes a risk that must be accepted and mitigated to support the Marketplace Day 1 operations."
In an exclusive interview with CNN last week, Sebelius said Obama didn't know of the problems with the ACA's website until after the troubled launch. This was despite the fact that insurance companies had been complaining and the site crashed during a pre-launch test run.
Earlier congressional questioning
Invited to testify last week, Sebelius put it off for travel related to efforts promoting enrollment in new health insurance exchanges under the health care reforms, Obama's most significant domestic policy achievement.
One of her lieutenants -- Marilyn Tavenner, who heads the Centers for Medicare and Medicaid Services unit that oversees the HealthCare.gov website -- became the first government official to face congressional questioning on Tuesday at a House Ways and Means Committee hearing.