WASHINGTON — The ongoing technical problems that have hampered enrollment in the online health-insurance exchanges resulted from the failure of a major software component, designed by private contractors, that crashed under the weight of millions of users last week, federal officials said Monday.

Todd Park, President Obama’s top technology adviser, said the failure occurred in the part of the website that lets people create user accounts at the beginning of the sign-up process. The crash prevented many people from viewing insurance options or accessing information on what federal subsidies might be available.

“At lower volumes, it would work fine,” Park said of the website, healthcare.gov. “At higher volumes, it has problems.”

The prime contractor for the exchange — CGI Federal, a unit of Montreal’s CGI Group — and the firm operating a “data services hub” for the government — Quality Software Services, a unit of the UnitedHealth Group — told Congress in September they were ready for enrollment to open Oct. 1.

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But in recent days, officials at the companies declined to answer questions about the problems. Both companies said they had passed operational-readiness reviews conducted last month by the federal government.

The Government Accountability Office, an investigative arm of Congress, said CGI had received $88 million for work on the federal exchange through March, while Quality Software Services had received $55 million for work on the data hub. The hub allows exchanges to get information about a person’s income and citizenship from the Internal Revenue Service, the Department of Homeland Security and other agencies.

As the engineers for the contractors struggle to recover from the website’s failures, officials said the partial shutdown of the federal government was also hampering efforts to carry out the health-care law and had slowed work on a federal insurance marketplace for residents of more than 30 states.

All insurers in the federal exchange have been assigned an account manager, the primary point of contact with the exchange. But many of the managers have been furloughed in the shutdown.