In Tennessee, the state insurance commissioner approved a 36 percent rate increase for the largest health insurer in the state’s individual marketplace. In Iowa, the commissioner approved rate increases averaging 29 percent for the state’s dominant insurer. Health insurance consumers logging into HealthCare.gov on Sunday for the first day of the Affordable Care Act’s third open enrollment season may be in for sticker shock, unless they are willing to shop around. Federal officials acknowledged on Friday that many people would need to pick new plans to avoid substantial increases in premiums.

Tens of thousands of people with modest incomes are at risk of losing health insurance subsidies in January because they did not file income tax returns, federal officials and consumer advocates say. Under federal rules, anyone who receives an insurance subsidy must file a tax return to verify that the person was eligible and received the proper amount of financial assistance based on household income.

The toll of failed co-op insurers, which were intended to challenge dominant companies that wield considerable power to dictate prices, has left about 500,000 customers scrambling to find health insurance for next year. A ninth co-op, which served Iowa and Nebraska, closed in February.

Federal investigators from the Government Accountability Office said on Thursday that they had discovered many errors in eligibility decisions under the Affordable Care Act that had led the government to pay for duplicate coverage for some people and an excessive share of costs for others.

The investigators said some people were getting subsidies for private insurance at the same time they were enrolled in Medicaid.