The U.S. Department Of Health and Human Services has opened an investigation into the cyberattack at UnitedHealth Group's Change Healthcare unit that disrupted pharmacy refills and insurance claims processing.

The investigation, announced on Wednesday, is the first government probe into the Feb. 21 cyberattack that has jolted the U.S. health system as the division processes about 50% of medical claims in the country.

The probe will focus on identifying the extent of the possible breach, and compliance by UnitedHealth and the unit with federal law that protects sensitive patient health information, the HHS said.

UnitedHealth said it would cooperate with the investigation.

"Our immediate focus is to restore our systems, protect data and support those whose data may have been impacted," it said.

The full extent of the data breach remains unknown, and UnitedHealth has said it was still investigating.

UnitedHealth has blamed the hack to the "Blackcat" gang, a notorious ransomware group that has a history of disruptive attacks.

In a message posted to, and then quickly deleted from their darknet site, the hackers said on Feb. 21 that they stole millions of sensitive records, including medical insurance and health data, from the company.

  - Reuters