Aflac said Friday that it experienced a cybersecurity incident last week that may have impacted files containing social security numbers, health information and other personal information.

Insurance giant Aflac identified “suspicious activity” in its U.S. network, which initiated its cybersecurity protocols. Aflac said it was able to stop the attack within hours, and its systems were not affected by ransomware.

The insurer said it’s in the early stages of a review of the incident, and so far is unable to determine the total number of affected individuals.