ATLANTA — Georgia Power says an unauthorized person accessed limited information through its online customer portal, affecting about 400,000 accounts associated with parent company Southern Company. About 300,000 of those accounts belong to Georgia Power customers. The utility said it recently detected suspicious activity, investigated and found that customer names, mailing addresses, phone numbers, email addresses and the last four digits of Social Security numbers were exposed. Georgia Power has not said exactly which information was accessed for individual customers. The company said it took steps to stop the activity and notified law enforcement. It also said its investigation found no evidence that unauthorized access is continuing. Georgia Power is contacting affected customers and providing free credit monitoring. Customers should be alert for unexpected messages or calls claiming to be from the utility, particularly requests for personal or financial information. The company has not released further details about how the intruder gained access or when the incident occurred.