UnitedHealth Group said on Monday the cyberattack at its technology unit led to a breach of health and personal information, which could cover a 'substantial proportion' of Americans.
The company, however, did not disclose the number of people impacted by the February cyberattack at the Change Healthcare division.
It said it would take several months before enough information to identify impacted individuals.
The company was monitoring the internet and dark web to determine if data has been published, along with external industry experts.
With about one in three U.S. patient records accessed by the unit's health technology offerings, the cyberattack sent shockwaves across the nation's healthcare system.
The hack at Change, a provider of healthcare billing and data systems and a key node in the U.S. healthcare system, disrupted payments to doctors and healthcare facilities nationwide.
The unit was breached on Feb. 21 by a hacking group called ALPHV, also known as "BlackCat."
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