UnitedHealth Group Inc. shares fell after a report that Republican Sen. Charles Grassley is launching an inquiry into the insurer’s Medicare billing practices.

The Senate Judiciary Committee probe is looking at UnitedHealth’s Medicare Advantage program, a privately administered version of the US Medicare health coverage plan for seniors and the disabled, the Wall Street Journal reported.

Grassley, an Iowa Republican, has long pushed for greater oversight of Medicare Advantage plans. The inquiry signals additional pressure on the company as Republicans in Congress weight a major tax and spending overhaul and the Trump administration scrutinizes federal spending. Grassley wrote that Medicare plans “continue to defraud the American taxpayer, costing them billions of dollars a year.”

His letter asked the company for extensive records about its system for recording patient diagnoses that affect payments in Medicare plans, including details about compliance programs, training manuals, and the use of artificial intelligence.

Shares of the health insurer fell as much as 2.5% Tuesday in New York. Shares of Humana Inc., another large Medicare insurer, also dipped as much as 2.4% before recovering.

“We welcome the opportunity to share the facts with Sen. Grassley,” UnitedHealth said in an emailed statement pushing back against the Wall Street Journal report. The company said Medicare Advantage plans are “highly regulated by the federal government” and said it holds itself “to the highest ethical standards of quality and integrity.”

(With assistance from Ike Swetlitz.)

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