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The soaring costs of Medicaid expansion in eight U.S. states – including Michigan, Ohio, New Hampshire and Illinois, is under investigation by a U.S. Senate committee.

Sen. Ron Johnson, chairman of the Senate Homeland Security and Governmental Affairs Committee, sent letters Wednesday to the governors of the eight states asking about the increasing costs that "go far beyond" initial projections.

“Federal Medicaid expenditures totaled $246 billion in fiscal year 2009, increased to $299 billion in fiscal year 2014 and are projected to rise 96 percent to $588 billion by 2025," Johnson, R-Wisconsin, said. "A primary cause of this increase is the ACA Medicaid expansion. Current CMS and other data show original Medicaid expansion per-enrollee spending and overall enrollment projections were significantly understated."

The letter to Illinois Gov. Bruce Rauner says the costs of Medicaid expansion in Illinois exceed initial estimates by 90 percent.

"Costs per enrollee are also surging in Illinois, going from $1,867 in 2014 to $5,854 in 2015 – a 214 percent single-year increase," Johnson wrote. "I am seeking to better understand these rising costs and higher-than-expected enrollment, especially in states where costs or enrollment are increasing especially quickly."

Ohio's new enrollees exceeded estimates by 60 percent and the cost per enrollee surged by 30 percent, Johnson wrote.

In New Hampshire, total Medicaid expenses increased by 253 percent between 2014 and 2015, he said.

And in Michigan, costs rose 72 percent and the cost per enrollee increased 86 percent.

Johnson asked the governors to provide information about new Medicaid enrollees in each state in 2016 and 2017, an explanation for why enrollment is increasing significantly faster than expected, why the per-enrollee costs increased significantly, and what each states' eligibility thresholds are.

The other states that received similar letters are California, West Virginia, New York and Hawaii.