Former CEO and current chairman Stephen Hemsley is stepping back into the top job following the abrupt resignation of Andrew Witty, UnitedHealth said.

UnitedHealth Group Chief Executive Officer Andrew Witty stepped down, effective immediately, "for personal reasons", the company said Tuesday. “Leading the people of UnitedHealth Group has been a tremendous honor as they work every day to improve the health system, and they will continue to inspire me,” Witty said in a statement shared by UHG.

As CEO for more than a decade, Stephen Hemsley built up UnitedHealth Group, now a $400 billion healthcare behemoth. With his return to the top job, the company will learn whether his playbook still works.

A group of investors this week filed a proposed class-action lawsuit against UnitedHealth Group, alleging that the company withheld information following the murder of UnitedHealthcare CEO Brian Thompson last December.

UnitedHealth Group (NYSE: UNH) today announced the appointment of Stephen J. Hemsley as its chief executive officer, effective immediately, following Andrew Witty’s decision to step down as CEO for personal reasons.

The lawsuit, which seeks class action status, alleges that UnitedHealth and its executives didn’t properly warn investors about how the killing of UnitedHealthcare CEO Brian Thompson would affect its business.