UnitedHealth Group abruptly named a new chief executive on Tuesday, marking a swift leadership change months after the high-profile fatal shooting of UnitedHealthcare’s CEO in December.

Andrew Witty has resigned as CEO of UnitedHealth, citing vague “personal reasons,” the company said. Taking over is Stephen J. Hemsley, who previously led the company from 2006 to 2017. Hemsley will also continue to serve as board chairman. Witty isn’t exiting entirely—he’ll shift into a senior advisory role, according to UnitedHealth’s statement.

Stephen J. Hemsley.

The company, already under scrutiny for its role in the broader health insurance industry, has faced intensifying criticism over the past year. Its stock has also taken a beating, dropping nearly 15% on Tuesday alone. UnitedHealth, a Dow Jones component, now trades around $321—a stark fall from its November peak of $630.73. A Justice Department investigation into the company’s business practices continues to loom.

UnitedHealth also announced it’s suspending its 2025 financial forecast. The company pointed to unexpectedly high medical costs among new Medicare Advantage enrollees and a wider range of benefits offered in the first quarter as major factors. Still, it expects to bounce back in 2026.

This leadership shake-up comes in the shadow of a tragedy. Brian Thompson, the former UnitedHealthcare CEO, was shot and killed last December in a targeted attack near an investor event in Manhattan.

Luigi Mangione, now 27, was arrested at a McDonald’s in Altoona, Pennsylvania, after a five-day manhunt. He is facing both federal and state charges in Pennsylvania and New York. Mangione has pleaded not guilty to state charges including murder and terrorism, and federal charges including stalking and murder. If found guilty on the federal charges, Mangione could face the death penalty.