(Reuters) -UnitedHealth Group ( UNH ) said on Wednesday that its unit Change Healthcare's pharmacy network was back online, weeks after a cyberattack.

Change Healthcare processes about 50% of medical claims in the United States for around 900,000 physicians, 33,000 pharmacies, 5,500 hospitals and 600 laboratories, and the hack had a knock-on effect across the country's healthcare system which depends heavily on insurance.

As of March 13, all major pharmacy and payment systems are up and more than 99% of pre-incident claim volume is flowing, UnitedHealth ( UNH ) said.

The company, however, added that its teams are working on some pharmacies that are still offline.

Earlier in the day, the U.S. Department Of Health and Human Services opened an investigation into the Feb. 21 cyberattack.

UnitedHealth ( UNH ) had blamed the hack on 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.

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

UnitedHealth ( UNH ) has also been hit with at least six class action lawsuits accusing it of failing to protect millions of people's personal data following the hack, with more lawsuits likely to come.