The company said its experts were working with law enforcement authorities and third-party consultants to gauge the impact on its customers and patients.
The statement confirms a Reuters report on Monday. UnitedHealth had initially blamed a "suspected nation-state associated cybersecurity threat actor" for the disruption.
Blackcat has not returned repeated messages from Reuters, seeking a comment, including a message sent on Thursday asking about UnitedHealth's statement.
The problems began last week after hackers gained access to UnitedHealth's Change Healthcare unit, a vital lynchpin in the complex U.S. system for making and clearing insurance claims.
It affected electronic pharmacy refills and insurance transactions, particularly among independents, with some reverting to paper transactions.
