A third of Americans may have had their personal data swept up in a February ransomware attack on a UnitedHealth Group subsidiary that disrupted pharmacies across the US, UnitedHealth CEO Andrew Witty estimated in testimony to Congress on Wednesday.

WASHINGTON — Capitol Hill lawmakers from both parties on Wednesday grilled UnitedHealth Group’s CEO over the largest-ever cyberattack on the U.S. health care industry, which has crippled payments to providers and pharmacies and left millions of patients clueless about whether their information is now on the dark web.

Hackers who breached UnitedHealth's tech unit in February potentially stole a third of Americans' data, the largest U.S. health insurer's CEO told a Congressional committee on Wednesday. Two Congressional panels grilled CEO Andrew Witty about the cyberattack on the company's Change Healthcare unit,