WASHINGTON — Federal health officials were responsible for the problem-pocked start of HealthCare.gov last year because of poor planning and lax oversight of outside contractors, according to government investigators who warned that “significant risks remain” that some Americans could again have trouble buying coverage in the federal health insurance marketplace this fall.

Such management failures are the central conclusion of the first report issued by the Government Accountability Office as part of a wide-ranging appraisal of the reasons the computer system was not ready when the marketplace opened in October.

The initial slice of the GAO’s work focuses on the main contractors the government hired to build HealthCare.gov, the website for the federal insurance exchange created under the Affordable Care Act. In particular, the report examines the shepherding of the contractors by the Centers for Medicare and Medicaid Services (CMS), the branch of the Department of Health and Human Services responsible for developing the marketplace.

Building “a first-of-its-kind marketplace” was certain to be a complex undertaking, the investigators conclude in the report, made public on Wednesday. But agency officials aggravated the situation by allowing too little time for the work; changing the directions it gave the main contractor, CGI Federal; and not scrutinizing the contractor’s progress, the investigators found. The results, the GAO says, were “significant cost increases, schedule slips” and delays. Between September 2011 and February of this year, the cost for building the marketplace ballooned from $56 million to $209 million. Building HealthCare.gov, the report said, had cost $840 million as of earlier this year, the GAO found.