Hospital operator Ascension told Maine's state attorney general on Friday that nearly 5.6 million people were affected in a ransomware attack that hit it earlier this year.
The company said an unspecified amount of medical data - including patients' medical records, lab tests and insurance information - was compromised.
Cybercriminals use ransomware to paralyze computer networks and extort a payment, typically in cryptocurrency. Many also steal data for added leverage.
Hospitals and healthcare providers - whose data is particularly sensitive and whose operations are especially critical - have regularly been targeted.
The hack disrupted clinical operations, Reuters reported at the time.
Founded as a Catholic nonprofit in 1999, the Ascension network has about 134,000 associates, 35,000 affiliated providers and 140 hospitals, serving 19 states and the District of Columbia.
This year a particularly damaging intrusion at UnitedHealth affected the information of 100 million people.
