(Reuters) — The Obama administration launched its troubled health care insurance website after only a minimum of crucial systemwide testing, despite contractors warning officials repeatedly about performance risks, a congressional panel heard on Thursday.

Witnesses said the administration did not conduct end-to-end testing of the system's technology backbone until just two weeks before one of the lynchpins of President Barack Obama's landmark health care policy opened to consumers on Oct. 1.

At a U.S. House of Representatives oversight committee hearing, contractors also blamed the administration for a last-minute design change that has been identified as a flaw responsible for leading millions of visitors into system bottlenecks.

Julie Bataille, a spokeswoman for the Centers for Medicare and Medicaid Services, the agency implementing the online marketplace, acknowledged the contractors' testimony.

"Due to a compressed time frame, the system wasn't tested enough," Ms. Bataille said. "What's important to realize is that we are putting in place a much more robust performance testing system now."

She also told reporters that in-house "business decisions" prompted CMS to require online visitors to create accounts before shopping for health plans and prompted the agency to assume the key role of system integrator for Healthcare.gov.

The glitches, delays and errors that have characterized the website are a growing concern for Republicans and Democrats alike. The administration is racing to solve the problems in time for millions of uninsured Americans to enroll for coverage and begin receiving health benefits from Jan. 1, as stipulated by the 2010 Patient Protection and Affordable Care Act, commonly called "Obamacare."

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CMS said on Thursday that about 700,000 applications have been submitted so far for U.S. health care coverage through the exchanges.

"We would certainly have liked to see as much time as possible for end-to-end testing," said Andrew Slavitt, executive vice president for the parent of CGI Federal and Quality Software Services Inc., a unit of health insurer UnitedHealth Group Inc.

QSSI produced the federal data hub and a software tool for creating online consumer accounts, which was at the center of early logjam problems. The design change involved turning off anonymous browsing and requiring online visitors to create accounts before researching health plan information and determining their eligibility for federal subsidies to help pay premiums.

Mr. Slavitt and Cheryl Campbell, senior vice president for CGI Federal, the main contractor, said months of testing would have been preferable for a big new information technology system, but that the testing schedule for Healthcare.gov was determined by CMS.

The rollout went ahead after QSSI said it made CMS aware of its concerns throughout the system's development, but Mr. Slavitt was not aware of any response from the agency.

"The concerns that we had, which were mostly related to testing and the inability to get as much testing as we'd like — we expressed all of those concerns and risks to CMS," he said. "My understanding is they understood those and were working on them. But I don't know further."

Healthcare.gov is the online Web portal for a federal health insurance marketplace that contractors described as one of the most complicated large-scale IT systems in existence.

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The 4.5-hour hearing before the House Energy and Commerce Committee marked the first full-length public airing over the problematic rollout, giving lawmakers the chance to piece together what went wrong at the beginning of a six-month enrollment period expected to draw at least 7 million enrollees for 2014.

"This is not about blame — this is about accountability, transparency and fairness for the American public. The broken promises are many," said Rep. Fred Upton, R-Mich., who chairs the committee.

"We still don't know the real picture, as the administration appears allergic to transparency and continues to withhold enrollment figures," Rep. Upton said.

The U.S. Department of Health and Human Services and the White House have largely declined to disclose information about the problems plaguing the system. It cost nearly $400 million to build, according to the Government Accountability Office.

Outside the hearing, Rep. Darrell Issa, R-Calif., and Sen. Lamar Alexander, R-Tenn., raised the possibility of a subpoena to compel the administration to release documents related to the cost and performance of Healthcare.gov after its first nine days of operation. They asked HHS for the information by Oct. 23, but the department did not comply.

In a letter to HHS Secretary Kathleen Sebelius, the pair said Rep. Issa's House Oversight and Government Reform Committee would consider using "compulsory process" if the information were not provided by 5 p.m. EDT (2100 GMT) on Monday. HHS had no immediate response.

Most of the criticism has come from Republicans who have long opposed the law, President Obama's most significant domestic policy achievement, as an unwarranted expansion of the federal government.

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Rep. John Shimkus, R-Ill., demanded the names of administration officials involved in the decision-making: "I would venture to guess the regular bureaucrats did their job. The political appointees manipulated."

Democrats largely dismissed the Republican rhetoric as partisan politics, saying the committee's goal should be to "fix, not nix" the law.

"The Affordable Care Act is an enormous success with one obvious exception: It has a poorly designed website," said Rep. Henry Waxman, D-Calif., the top Democrat on the Energy and Commerce Committee.

But some Democrats also expressed disapproval. Rep. Anna Eshoo, D-Calif., said it was "a lame excuse" for contractors and the administration to claim that high volumes of visitors were responsible for problems.

"Taxpayers paid you a lot of money, and you're essentially saying to us that everything's alright when it's not," Rep. Eshoo told CGI's Ms. Campbell after she assured lawmakers that the problems would be fixed in time for consumers to enroll for benefits beginning Jan. 1.

"We anticipate that the system, as we have seen, is improving day over day," Ms. Campbell said at the hearing. CGI Federal is a unit of Canada's CGI Group Inc.

The House oversight focus will switch next week to the administration as Ms. Sebelius appears before Rep. Upton's panel and her lieutenant, CMS Administrator Marilyn Tavenner, testifies before the House Ways and Means Committee.