The sign-up website had been taken down briefly Friday, with consumer interest surging.
A recent analysis for The Associated Press by the performance-measurement firm Compuware found that the government site runs slow compared with health insurance industry peers.
With long lines reported at centers offering in-person assistance to enroll, HHS Secretary Kathleen Sebelius was leading the deadline day sign-up effort. Sebelius planned to spend much of the day Monday working out of the department's TV studio, conducting interviews by satellite with local media stations around the country.
Although March 31 was the last day officially to sign up, millions of people were potentially eligible for extensions granted by the administration for various reasons.
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That includes consumers who had begun enrolling by the deadline but didn't finish, perhaps because of errors, missing information or website glitches. The government says it will accept paper applications until April 7 and take as much time as necessary to handle unfinished cases on HealthCare.gov. Rules may vary in states running their own insurance marketplaces.
The administration is also offering special extensions to make up for all sorts of problems that might have kept people from getting enrolled on time: Natural disasters. Domestic abuse. Website malfunctions. Errors by insurance companies. Mistakes by application counselors.
To seek a special enrollment period, contact the federal call center, at 1-855-889-4325, or the state marketplace and explain what happened. It's on the honor system. If the extension is approved, that brings another 60 days to enroll.
—By The Associated Press