WASHINGTON — Republicans in the House of Representatives are making plans to investigate the disastrous Oct. 1 launch of the federal health insurance marketplace established under the Affordable Care Act.

The House Energy and Commerce Committee has asked Health and Human Services Secretary Kathleen Sebelius and the federal contractors involved in the rollout to explain why things have gone so badly after their earlier testimony indicated that the marketplace information technology would run smoothly.

The federal marketplace, Healthcare.gov, was supposed to provide a one-stop site for users in 36 states to browse, compare and enroll in qualified health plans.

But millions of users and numerous software problems overwhelmed the site shortly after the enrollment period for 2014 coverage began.

In subsequent days, the site was shut down temporarily for repairs, which have continued since the problems first surfaced.

Two weeks later, site navigation has improved but delays and malfunctions continue to dog the system, making it difficult for users to establish personal accounts and obtain federal subsidies to offset the cost of coverage.

Several government reports had warned that testing for the marketplace was months behind schedule and the data hub might not get final approval until days before the launch.

“Despite the widespread belief that the administration was not ready for the health law’s Oct. 1 launch, top officials and lead IT contractors looked us in the eye and assured us all systems were a go,” Rep. Fred Upton, R-Mich., the chairman of the House Energy and Commerce Committee, said in a recent statement. “The American people deserve to know what caused this mess.”