(Reuters) - UnitedHealth Group ( UNH ) said on Wednesday its unit Change Healthcare's pharmacy network was back online, weeks after a cyberattack had a knock-on effect across the country's healthcare system that depends heavily on insurance.

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.

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.

Separately, UnitedHealth Group ( UNH ) said it has identified the source of the intrusion and established a way to restore the affected systems.

A thorough forensic analysis with Alphabet's cybersecurity unit Mandiant and Palo Alto Networks is underway, UnitedHealth ( UNH ) said, without providing additional details about the source of the data breach.

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.

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.