The massive cyberattack on Change Healthcare has potentially compromised the personal and protected health information of an untold number of Americans, according to parent company UnitedHealth Group.

The personally identifiable information and protected health information for “a substantial proportion of people in America” might have been among the compromised files, UnitedHealth said Monday in a press release announcing preliminary findings from its investigation.

Change Healthcare, a unit within UnitedHealth’s Optum, processes 15 billion financial and other transactions annually for doctors, hospitals, pharmacies, and other health entities in the United States. The cyberattack in February lowed health claims processing to a crawl nationwide, which has meant long delays in payment for many providers, including those relying on federal Medicare payments.