Health and Human Services Secretary Sylvia M. Burwell greets Dr. Abraham Alecozay as she tours a health insurance enrollment event at Southwest General Hospital in San Antonio late last month. (Eric Gay/AP)

Nearly 13 million people are now signed up for health plans through the Affordable Care Act’s insurance marketplaces, according to federal figures released Thursday, yielding a modest gain of about 1 million customers for 2016.

During the third open-enrollment season, 9.6 million consumers chose coverage in the 38 states that rely on HealthCare.gov, the website for the federal marketplace. An additional 3.1 million selected plans in states that run their own insurance exchanges.

In announcing results of the three-month enrollment window that closed on Sunday, Health and Human Services Secretary Sylvia Mathews Burwell portrayed the figures as a success. “The totals exceed our expectations,” she said during a call with reporters.

Other health policy experts said the numbers were in line with recent government predictions that growth in coverage through the law’s marketplaces is slowing sooner than originally expected.

Of those who signed up through HealthCare.gov, 4 million people — about 2 in 5 — are new customers, the figures show. But it is not clear how many are in the niche group that HHS targeted in its recent outreach efforts: the approximately 10.5 million Americans who remained uninsured despite being eligible to buy insurance through the ACA exchanges.

Even so, the latest numbers provide important signs of how the exchanges, a major feature of the 2010 health-care law, are functioning now that they are no longer new.

The figures show that most repeat customers heeded warnings by HHS officials that they should return to HealthCare.gov to shop for the best available health plan if they wanted to avoid sticker shock. On the federal exchange, nearly 7 in 10 repeat customers shopped for coverage rather than let the computer system renew them automatically, and about three-fifths of those switched plans. Last year, nearly half let the computer system auto-enroll them.

And this time, more consumers picked health plans during the first two months of the sign-up period, in time to be insured on Jan. 1. In contrast to the past two years, there was little of the last-minute surge of people rushing to sign up during the final days and hours of the enrollment period.

The latest results show that the exchanges are making relatively little headway in attracting a group of customers that insurers consider vital to their financial well-being: young adults who tend to be healthy and thus cost relatively little. In the federal marketplace, 2.7 million people ages 18 to 34 signed up for coverage — 28 percent of the total, the same proportion as last year. Early on, Obama administration officials said the exchanges would work well if slightly more than one-third of enrollees were young adults.

Revealing as the figures are, they remain an early snapshot. They include little detail about states with their own marketplaces. And the federal exchange’s tally includes everyone who chose — or was assigned to — a health plan, whether or not they begin to pay the monthly premiums that put their coverage in effect.

In each of the past two years, enrollment has slipped as the months go by. At the end of the 2015 enrollment season, 11.7 million people were signed up, a number that dropped to 9.3 million by early fall. HHS officials have not released the year-end tally for 2015.

The new figures suggest that HHS officials were realistic in the fall when they forecast that enrollment by the end of this year would be about 10 million — about half of what congressional budget analysts had forecast. Last week, the budget analysts lowered their own expectations, saying that, on average, 13 million people would have marketplace health plans throughout this year.