Insurance expansion in the early stages of the Affordable Care Act’s implementation boosted coverage for ex-prisoners but still left substantial gaps among a population with high rates of mental illness and chronic diseases such as hepatitis and diabetes, new research shows.

As expected, the health law’s 2014 expansion of Medicaid coverage for the poor was linked to higher insurance rates among those on parole or probation, and others with recent involvement with the criminal justice system.

The large majority of ex-prisoners are eligible for Medicaid in states that opted to expand it, experts say.

Another portion of the health law was also associated with substantial, and perhaps surprising, increased coverage for released prisoners.

After 2010, when the law required employer medical plans to cover dependents up to age 26, the portion of young adult ex-inmates without insurance fell from 40 percent to 32 percent.

Presumably many of them got coverage through their parents’ job-based plans, said Dr. Tyler Winkelman, a researcher at the University of Michigan and the lead author of the study, published Monday in the Journal of General Internal Medicine.

“It’s a fascinating finding,” he said. “This could be a really important way that justice-involved individuals get insurance coverage, because so many of them are younger than 26.”

Policymakers see the health law as a way to connect millions of ex-prisoners to care, cut recidivism and save money by reducing expensive visits to hospital emergency rooms, which are often the provider of last resort for the uninsured.

But coverage is still far from universal. Nineteen states haven’t expanded Medicaid. Among those that have, prisons and jails are doing a spotty job of enrolling released prisoners.

As of 2014, the first year of the Medicaid expansion, 30.7 percent of ex-inmates and other justice-involved people were still uninsured, according to the researchers’ analysis of a national survey on drug use and health. The rate had previously hovered around 40 percent.

Newer figures may show continued increases in coverage among released prisoners, Winkelman said.

Several states have expanded Medicaid since 2014. Plus, he said, “I would assume that as more jails and prisons are coming online with programs, they’ll enroll more people.”