President Barack Obama's Republican opponents have long opposed his flagship health insurance reform, and have gleefully seized on the failed launch of its online sign-up page.
Executives from firms contracted by the Department of Health and Human Services said the site's debut had been hit by technical complexities and an overwhelming response by consumers.
But they also testified that, while ideally there should have been months of testing before launching such a complex system, HHS waited just two weeks before taking the website live on October 1.
"It would have been better to have more time," Cheryl Campbell, senior vice president of contractor CGI Federal, admitted to House of Representatives members in a testy hearing.
Andrew Slavitt, group vice president of contractor Optum, said the website's dry run only began in mid-September, adding: "Ideally, integrated testing would have occurred well before that date."
Even as they and others testified that many initial technical glitches they encountered have been resolved, lawmakers vented their anger over the deeply embarrassing rollout.
"This is not about blame, it's about accountability, transparency and fairness for the American public," said House Energy and Commerce Committee chairman Fred Upton, who said the problems are symptomatic of broader difficulties with the law.
"The broken promises are many," he said.
Upton said that in the months before the online debut, contractors and officials "looked us in the eye and assured us repeatedly that everything was on track -- except that it wasn't."
Fellow Republican Joe Pitts went further, decrying the botched rollout as "one of the biggest IT disasters in government history."
Obama is facing criticism for failures stemming from the debut of the site -- Healthcare.gov -- through which millions of Americans are expected to buy insurance.
Consumers have had trouble signing on, getting accurate cost estimates, and completing enrollment.
With the problems under the microscope, the White House moved to clear up confusion about the law.
Obamacare requires most Americans to have health insurance from 2014 or face a fine, but the White House said it will give an extra six weeks, until March 31, to obtain insurance before facing the penalty.
Thursday's hearing was the first of three over the next week arranged by House Republicans, who oppose the reforms and often used political rhetoric at the hearing to convey their distaste for Obamacare.
But over the course of four hours, frustration with the contractors bubbled over among even the law's most fervent backers.
"How soon will it take to repair these glitches?" demanded Democrat Eliot Engel.
Campbell said there was no "silver bullet" to fix the site, but "I have a team of people working around the clock trying to get this quickly resolved."
When Slavitt said the site's components were "overwhelmed" by early user demand, he earned a reprimand.
"That's really kind of a lame excuse," said congresswoman Anna Eshoo, scoffing at the idea that excessive volume would have crashed a $500 million operation.
"This is the 21st century," added Eshoo, a Democrat representing the California tech hub of Silicon Valley. "Amazon and e-Bay don't crash a week before Christmas."
But when Republican Joe Barton accused officials of violating privacy laws by transferring private health data through the website, Democrat Frank Pallone came to their defense, insisting the system used no such consumer information.
"Once again my Republican colleagues are trying to scare everybody," he boomed, decrying the criticisms as an effort to shame the administration into dismantling Obamacare.
Contractors argued the site's complexity -- it links millions of customers to federal and private databases like those of the Social Security Administration and 170 insurance firms -- was a key factor in its poor start.
A main problem arose at the entryway, where the requirement for consumers to create a secure account caused a "bottleneck" that prevented many users from accessing the online marketplace.
The problems may stem from a last-minute decision by HHS to require users to register before they could window shop.
"We don't know why the decision was made," said Slavitt, who added his company -- which designed the gateway -- learned of the change just 10 days before the site went live.
Obama's Health Secretary Kathleen Sebelius, who has resisted critics' calls to resign over the poor rollout, is expected to testify next week.