Late last week, Marriott International disclosed one of the largest data breaches in history, announcing approximately 500 million Starwood Hotels and Resorts’ customers’ data was compromised.

According to Marriott, the breach became clear when Marriott received an alert from one of its software security tools Sept. 8, indicating an attempt to access Starwood guest reservation information. A brief consultation with security experts revealed the Starwood network has been compromised since 2014. Marriott discovered certain data was copied and encrypted by an unauthorized party.

On Nov. 19, Marriott discovered the copied data was Starwood guest reservation information. As of press time, Marriott did not completely finish sifting through the stolen information. For about 327 million guests, the data includes a combination of “name, mailing address, phone number, email address, passport number, Starwood Preferred Guest (“SPG”) account information, date of birth, gender, arrival and departure information, reservation date, and communication preferences.” For some, the stolen information included payment card numbers and expiration dates, but the payment information was encrypted, according to Marriott.

“Marriott deeply regrets this incident happened. From the start, we moved quickly to contain the incident and conduct a thorough investigation with the assistance of leading security experts. Marriott is working hard to ensure our guests have answers to questions about their personal information with a dedicated website and call center,” Marriott said in its response.

In order to combat the breach and make customers as comfortable as possible in dealing with it, Marriott set up a dedicated call center to answer questions; set up an email notification system; and provides for guests to enroll in a free WebWatcher service for one year.

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