Updated

The contractors behind the troubled ObamaCare website are planning to spread the blame around during highly anticipated testimony Thursday morning on Capitol Hill.

While Health Secretary Kathleen Sebelius is skipping the hearing in order to visit an ObamaCare call center in Phoenix, representatives from four contracting companies are expected to testify before the House Energy and Commerce Committee. They are acknowledging problems with the site, but according to prepared testimony also pointing a finger back at the Obama administration.

A top executive with CGI Federal, one of the contractors paid millions to create the ObamaCare website, said the Department of Health and Human Services "serves the important role of systems integrator or `quarterback' on this project and is the ultimate responsible party for the end-to-end performance."

Senior Vice President Cheryl Campbell also said in her prepared remarks that “no amount of testing” could have prevented the site’s problem-plagued start.

The House Energy and Commerce Committee hearing is the first since the site’s disastrous Oct. 1  launch -- marked by crashes, slow response times and its inability to let customers make purchases. Contractors will likely face tough questioning from lawmakers on both sides of the aisle, though the witnesses appear to be implicating government officials overseeing the project.

Prepared testimony from contractor Optum/QSSI blamed in part a "late decision" to require customers to register before browsing for insurance, which could have helped overwhelm the registration system.

"This may have driven higher simultaneous usage of the registration system that wouldn't have occurred if consumers could window-shop anonymously," said Andy Slavitt, representing QSSI's parent company.

In her prepared testimony, Campbell argued that with a system “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.”

“No amount of testing within reasonable time limits can adequately replicate a live environment of this nature,” she adds, according to the prepared testimony.

Campbell said issues with HealthCare.gov have continued to improve over the past two weeks, but additional challenges are occurring as more users get past the registration screen and buy insurance -- including “data assurance issues.”

She said such problems can be fixed through “tuning, optimization and application improvements.” However, Campbell also said that the buck ultimately stops with the Department of Health and Human Services, which she said "serves the important role of systems integrator or "quarterback' on this project and is the ultimate responsible party for the end-to-end performance."

The testimony comes as the Obama administration offered new details and explanations on Wednesday. The administration said the system didn't get enough testing, especially at a high user volume. It blamed a compressed time frame for meeting the Oct. 1 deadline to open the insurance markets. Basic "alpha and user testing" are now completed, but that's supposed to happen before a launch, not after.

The Health and Human Services explanation identified some bugs that have gotten little outside attention.

For example, technical problems have surfaced that are making it hard for people to complete the application and plan-shopping functions. That's a big concern because those stages are further along in the signup process than the initial registration, where many consumers have been getting tripped up. The problems are being analyzed and fixes are planned, the department said.

The explanation, posted online in a department blog and accompanying graphic, identified other broad areas of problems and outlined fixes underway but in most cases incomplete:

-- Unexpectedly high consumer interest that overwhelmed the system in its initial days. Equipment has been added to handle the load and system design has been improved. More fixes are in progress.

-- Lack of a way for consumers to browse their health plan options without first having to set up a user account. A partial fix is in place.

-- Incorrect or duplicate information in enrollments is being delivered to insurance companies. Some software fixes that should help address the issue have been completed, others are underway.

-- Difficulties for consumers trying to create user accounts, including drop-down menus that didn't work. Design changes and software fixes should address the situation.

On Thursday, the House Energy and Commerce Committee will line up contractors, including CGI Federal, the lead developer of the website, and QSSI, which designed a back-room operation known as the federal data services hub. The hub is integral to verifying applicants' personal information and income details, and the administration says it is working as designed.

The Associated Press contributed to this report.