Many health policy experts believe that, in the two years since the marketplaces opened, they already have attracted the people who are easiest to enroll. (Andrew Harrer/Bloomberg)

Nearly 1 in 4 of the Americans who picked a health plan this year through the Affordable Care Act’s insurance marketplaces have dropped or lost their coverage, according to new federal data.

A report, released Tuesday by federal health officials, shows that 9.9 million people were paying, as of the end of June, for health plans they had gotten through the federal and state-run insurance exchanges created under the health-care law.

Those people reflect a decline from the roughly 12.7 million consumers who signed up for a 2015 health plan. The vast majority signed up during an open enrollment period that ended in February and the rest through special enrollment circumstances since then.

The figures also mean that health plans sold on the exchanges are covering slightly fewer people as the year goes on. Between the end of March and the end of June, the number of people with active coverage dipped by about 300,000.

Still, taken together, the data released on Tuesday suggest that insurance through the exchanges is on target to reach the level the Obama administration has predicted for the end of this year.

Yet they also show the challenges ahead. For next year, congressional budget analysts are estimating that 21 million Americans will have health insurance through the exchanges — more than double the enrollment now.

Many health policy experts think that, in the two years since the marketplaces opened, they already have attracted the people who are easiest to enroll. Elizabeth Carpenter, a vice president of the consulting firm Avalere Health, said that the Congressional Budget Office has been assuming that sign-ups for 2016 will surge, because financial penalties will increase under a part of the law that requires most Americans to carry health insurance.

“The question is,” Carpenter said, “given where we are today, should we expect a slower ramp-up?”

When the exchanges began during the fall of 2013, the questions of how many people would buy coverage and how many would keep it constituted a big mystery about how the Affordable Care Act would work in practice. By the end of the first enrollment period, Obama administration officials were delighted that, despite crippling computer problems that at first hindered consumers trying to enroll, about 7 million Americans had chosen health plans.

But for more than a year after that, administration officials did not provide any information about how many of those people had paid at least their first month’s premium — or their share of it, since most were getting federal subsidies — so that their coverage would go into effect. The June figures, showing an “effectuated” enrollment of 10.2 million people, was the first clue.

The figures released Tuesday are the first update since then.

In all insurance — and particularly in coverage that individuals and families buy on their own — enrollment fluctuates. For the health plans sold through the exchanges, coverage is declining for several reasons, though neither federal officials nor insurers nor outside researchers have examined their relative importance.

In some cases, consumers did not begin to pay for their health plan, or stopped paying soon afterward. “Are people continuing to be able to afford insurance, and are they seeing value in their coverage?” Avalere’s Carpenter said. “That is a big question for exchanges long-term.”

Other people no longer need the individual coverage sold on the exchanges, because they or their spouse got a job with health benefits. Some people became poorer, so they joined Medicaid, while others turned 65 — the age to join Medicare.

In 423,000 cases as of the end of June, federal health officials cut off coverage to people they said had not provided proof that their citizenship or immigration status allowed them to buy health plans through an exchange.