House and Senate Republicans are investigating the glitch-ridden rollout of ObamaCare after administration officials downplayed technology flaws with the federal health overhaul website.

House Oversight Committee Chairman Darrell Issa, R-Calif., and Sen. Lamar Alexander, R-Tenn., sent a letter Thursday to Health and Human Services Secretary Kathleen Sebelius demanding answers to a series of questions about the technical failures.

The lawmakers are seeking details on the number of people who have successfully enrolled in the health insurance exchanges and information about all of the technical problems that have prevented enrollment and the cost of fixing those problems, according to a news release.

"We are concerned by recent comments to the media that the system suffers from architectural problems that need design changes," the lawmakers wrote. "We seek information about these problems as well as whether you still expect individuals to suffer a tax penalty if they do not purchase government-approved health insurance."

Consumers trying to create accounts multiplied the volume of online transactions that overwhelmed the federal health overhaul website last week, causing long waits and exasperation. The administration has promised "significant improvements" in accessing the website. 

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Republicans on the House Energy and Commerce Committee on Thursday sent letter to Sebelius and contractors involved with the creation of ObamaCare's IT systems, demanding information on implementation plans and contracts awarded by HHS.

The committee is requesting briefings with Obama administration officials and contractors as well as "all communications" between the contractors and HHS related to the Oct. 1 rollout and website functionality issues.

“Despite the widespread belief that the administration was not ready for the health law’s October 1 launch, top officials and lead IT contractors looked us in the eye and assured us all systems were a go,” Chairman Fred Upton, R-Mich., said in a statement. “Instead, here we are 10 days later, and delays and technical failures have reached epidemic proportions." 

There were 7 million visits to in the first two days. Federal health officials acknowledged problems beyond just high web traffic and technicians were adding equipment to expand the site's capacity and making software changes to reduce wait times.

Starting next year, the law requires virtually all Americans to have insurance or face a tax penalty, triggered after a coverage gap of three months. The penalty starts as low as $95 for 2014, but escalates in subsequent years. There are exemptions for financial hardship and other defined circumstances.

Consumers have until Dec. 15 to enroll for coverage that starts Jan. 1. They have until the end up March to sign up to avoid tax penalties.

The Associated Press contributed to this report.