Today is the deadline for most people to sign up for health insurance under the Affordable Care Act, the controversial healthcare-reform law.

More than 6 million Americans have already signed up for insurance coverage through online "marketplaces" created by the law, federal officials said on Thursday.

Earlier last week, the Obama administration said that Americans who've started applying for health insurance but can't complete the process by the March 31 deadline will be given an extension.

Administration officials said the extension was being offered partly out of concern that the federal online registration website, Healthcare.gov, could become overwhelmed as last-minute registrants scrambled to meet the original March 31 deadline or face a penalty in the form of a tax.

And that seemed to be the case Monday morning as the website struggled with high user traffic. Some visitors to HealthCare.gov were met with messages that the site was down for maintenance. At times, visitors were sent to a virtual waiting room -- a feature created to ease the strain on the site during periods of heavy demand, the Associated Press reported.

The troubled unveiling of HealthCare.gov last fall is still fresh in many people's memories, as computer glitches and software problems made the website almost unusable for long periods of time. Critics of the Affordable Care Act pounced on the troubled launch, which was deeply embarrassing to President Barack Obama. The health reform law, sometimes called Obamacare, is considered Obama's signature domestic achievement.

The federal website, which serves 36 states that do not operate their own registration websites, has been operating well for months, according to White House officials. But it had more than 1 million visitors each day several days last week. With the prospect of high visitor traffic leading up to March 31, the Obama administration said it wanted to avoid a repeat of last fall's problems with the website.

Administration officials did not specify how long the extended enrollment period would last.

GOP analysis says only two-thirds of people who signed up...

GOP analysis says only two-thirds of people who signed up through HealthCare.gov have paid premiums Credit: HealthDay

With some exceptions, people who are uninsured for most of 2014 may have to pay a penalty during next year's tax season under provisions of the controversial law. The maximum penalty for 2014 is $95 per adult and half of that for children (up to $285 for a family of three or more) -- or up to 1 percent of household income, whichever is greater.

According to the White House, one of the main objectives of the Affordable Care Act is to expand access to affordable health care options. The law led to the creation of the online marketplaces, or exchanges, where people in each state and the District of Columbia may compare health plans and sign up for coverage.

The Congressional Budget Office initially projected that 7 million people would sign up for health coverage in 2014. It later lowered its estimate to 6 million.

Once the enrollment deadline passes, most Americans won't have another chance to sign up for coverage until the next open enrollment period, which begins Nov. 15. Coverage purchased during that time won't take effect until 2015.

More information

Want to learn more about what income levels qualify for lower insurance costs? Visit HealthCare.gov.

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