WASHINGTON — The chief digital architect for the federal health insurance marketplace said Tuesday that 30 percent to 40 percent of the project was still being built.

The official, Henry Chao, made the assessment in testimony before a panel of the House Energy and Commerce Committee. Lawmakers expressed surprise that so much work remained to be done seven weeks after the federal website opened to the public.

Mr. Chao, the deputy chief information officer at the Centers for Medicare and Medicaid Services, said the government was still working on “back office systems,” including those needed to pay insurance companies that are supposed to provide coverage to millions of people under President Obama’s health care law.

“We have yet — we still have to build the financial management aspects of the system, which includes our accounting system and payment system and reconciliation system,” Mr. Chao said. These parts of the system, he said, are “still being developed and tested.”