UnitedHealth Group CEO Andrew Witty said Wednesday in testimony before the Senate Finance Committee that he is “deeply, deeply sorry” for a cyberattack that crippled much of the U.S. health-care system.

Addressing the hack of Change Healthcare, owned by a UnitedHealth subsidiary, Witty said he and the company will not rest “until we fix this.” The company has not yet said how many people had personal information compromised, but Witty said it “could cover a substantial portion of people in America.”