AT&T has begun notifying U.S. state authorities and regulators of a security incident after confirming that millions of customer records posted online last month were authentic.

In a legally required filing with Maine’s attorney general’s office, the U.S. telco giant said it sent out letters notifying more than 51 million people that their personal information was compromised in the data breach, including around 90,000 individuals in Maine.

AT&T — the largest telco in the United States — said that the breached data included customers’ full name, email address, mailing address, date of birth, phone number and Social Security number.

Leaked customer information dated back to mid-2019 and earlier, according to AT&T, but that the records contained valid data on more than 7.9 million current AT&T customers.

AT&T took action some three years after a subset of the leaked data first appeared online, which prevented any meaningful analysis of the data. The full cache of 73 million leaked customer records was dumped online last month, allowing customers to verify that their data was genuine. Some of the records included duplicates.

The leaked data also included encrypted account passcodes, which allow access to customer accounts.

Soon after the full dataset was published, a security researcher notified TechCrunch that the encrypted passcodes found in the leaked data were easy to decipher. AT&T reset those account passcodes after TechCrunch alerted AT&T on March 26 to the risk posed to customers. TechCrunch held its story until AT&T could complete the process of resetting affected customer passcodes.

AT&T eventually acknowledged that the leaked data belongs to its customers, including about 65 million former customers.

Companies experiencing data breaches that affect large numbers of people are required to disclose the incident with U.S. attorneys general under state data breach notification laws. In its notice filed in Maine, AT&T said it is offering identity theft and credit monitoring to affected customers.

AT&T has still not identified the source of the leak.