BETHESDA — A security breach at one of North America’s largest hotel groups, which includes the Sheraton, Westin, Starwood and Marriott brands, has exposed the personal information of as many as 500 million guests.

Marriott International, Inc. said Friday that credit card numbers and expiration dates of some guests may have been taken but it hasn’t yet determined if the payment information has been decrypted. It said that it learned during an investigation that began in September that there had been “unauthorized access” to the Starwood reservation data since 2014.

For as many as two-thirds of those affected, exposed data could include mailing address, phone number, email address, passport number, Starwood Preferred Guest account information, date of birth, gender, arrival and departure information, reservation date and communication preferences. For some guests, the information was limited to name and sometimes other data such as mailing address, email address or other information.

“We fell short of what our guests deserve and what we expect of ourselves,” CEO Arne Sorenson said in a prepared statement. “We are doing everything we can to support our guests, and using lessons learned to be better moving forward.”

Marriott said email notifications to those who may have been affected will begin rolling out Friday.

Marriott acquired Starwood Hotels in 2016. When their merger was announced in November 2015, Marriott had 54 million members of its loyalty program and Starwood had 21 million. Many travellers were members in both programs.

Starwood operates hotels under the names W Hotels, St. Regis, Sheraton Hotels & Resorts, Westin Hotels & Resorts, Element Hotels, Aloft Hotels, The Luxury Collection, Tribute Portfolio, Le Meridien Hotels & Resorts, Four Points by Sheraton and Design Hotels. Starwood branded timeshare properties are also included.

While the breach affected “approximately 500 million guests” who made a reservation at a Starwood hotel, some of those records could belong to people who had multiple stays.

Asked for more details on the 500 million number, Marriott spokesman Jeff Flaherty Friday said the company has not finished identifying duplicate information in the database.

Marriott said that there was a breach of its database in September, which had guest information related to reservations at Starwood properties on or before Sept. 10.

An internal security tool signalled a potential breach on Sept. 8, but the company was unable to decrypt the information that would define what data had potentially been exposed.

Marriott has had a rocky process of merging its computer system with Starwood computers. Members of both loyalty programs have complained about missing points, glitches with stays crediting to their accounts and problems with free nights earned from credit cards not appearing.

Sorenson said that Marriott is still trying to phase out Starwood systems.

Marriott has set up a website and call centre for anyone who thinks that they are at risk.