E-mails released late Thursday by House Republicans investigating the botched rollout of HealthCare.gov offer a snapshot of the chaos that unfolded behind the scenes within the Obama administration in the days and hours leading up to the troubled launch.

The 17 pages of notes released by the House Energy and Commerce Committee show tense communications between officials at the Centers for Medicare and Medicaid Services (CMS) and one of its contractors, with some worried that the system was not performing to expectations as the Oct. 1 launch date approached.

On Sept. 27 — three days before the site went live — David Nelson, acting director of the CMS Office of Enterprise Management, warned his colleagues of “defective code” and “inefficient queries.”

“We have not been successful in moving beyond 500 concurrent users filling applications without income verification,” he said.

But an official with CGI Federal, the main contractor on the project, took issue with the characterization of “defective code” and offered assurances that it is not unusual for such problems to turn up during performance testing.

On Sept. 29, U.S. Chief Technology Officer Todd Park expressed concern that there had not been enough testing. “Has the team run performance/diagnostic testing on the whole [exchange]” to determine exactly where the bottleneck was occurring? he asked. But he also praised the work underway, saying, “Massive kudos again for the incredible progress the team is making!”

The Web site froze shortly after launch, struggling under the pressure of thousands of people who flooded it during its early days. Since then, there have been a steady stream of reports noting that the performance testing referred to in the e-mails should have been completed months before launch.

Administration officials and another contractor have been working to fix the many flaws in the site. They say HealthCare.gov will be working smoothly for most people by the end of this month.

The e-mails were the latest batch of documents to be released by congressional Republicans, who have demanded reams of documents from the administration and the project’s contractors and have seized on the Web site’s issues as evidence that the overall health-care law is fatally flawed.

“The frenzied lead-up to October 1, coupled with the inadequate testing and numerous systems failures, reveal an administration that was not up to the job despite over three years to prepare,” Rep. Fred Upton (R-Mich.), chairman of the Energy and Commerce Committee, said in a statement.

In a statement, CMS officials said the e-mails represent “one piece of a number of ongoing discussions up to the launch of healthcare.gov on October 1st.”

White House officials accused Upton’s committee of “cherry picking” documents to serve a political agenda.

“While experts are working night and day to get HealthCare.gov fully functioning, House Republicans continue their obsession with sabotaging Obamacare,” White House spokesman Eric Schultz said in an e-mail. “They tried to block it, repeal it, overturn it, defund it, shut down the government over it, and now they are investigating it. This is a partisan political strategy that hasn’t worked in the past, and it’s not going to work now.”

Juliet Eilperin contributed to this report.