The prime contractor, CGI Federal, had long before concluded that the administration was blindly enamored of an unrealistic goal: creating a cutting-edge website that would use the latest technologies to dazzle consumers with its many features. Knowing how long it would take to complete and test the software, the company’s officials and other vendors believed that it was impossible to open a fully functioning exchange on Oct. 1.

Government officials, on the other hand, insisted that Oct. 1 was not negotiable. And they were fed up with what they saw as CGI’s pattern of excuses for missed deadlines. Michelle Snyder, the agency’s chief operating officer, was telling colleagues outright, “If we could fire them, we would.”…

The online exchange was crippled, people involved with building it said in recent interviews, because of a huge gap between the administration’s grand hopes and the practicalities of building a website that could function on opening day.

Vital components were never secured, including sufficient access to a data center to prevent the website from crashing. A backup system that could go live if it did crash was not created, a weakness the administration has never disclosed. And the architecture of the system that interacts with the data center where information is stored is so poorly configured that it must be redesigned, a process that experts said typically takes months. An initial assessment identified more than 600 hardware and software defects — “the longest list anybody had ever seen,” one person involved with the project said.