Frontier Communications has revealed that information for over 750,000 customers — including full names and Social Security numbers — was exposed in a data breach following a cyberattack on April 14th. Hackers claim to have even more and will release it unless Frontier pays a ransom.

The attack enabled hackers to access 751,895 customers’ personal data on Frontier’s systems according to a sample of the notice Frontier submitted to the Office of the Maine Attorney General. Frontier has notified impacted customers and provided them with one year of free credit monitoring and identity theft services, but says it “does not believe” customer financial information was exposed in the breach.

Bleeping Computer reports that the RansomHub extortion group claimed responsibility for the attack on June 4th and is threatening to leak the 5GB of customer data it allegedly stole unless Frontier responds to their demands by June 14th. The group claims the stolen dataset contains information belonging to two million Frontier customers, including their full name, physical address, date of birth, social security number, email address, credit score, and phone number.

Frontier says it’s bolstered its network security following the attack and notified both regulatory authorities and law enforcement. A securities filing reveals that the company was forced to shut down some of its systems to contain the incident.