Hotel group Marriott International has confirmed another data breach, with hackers claiming to have stolen 20 gigabytes of sensitive data including guests’ credit card information.

The incident, first reported by Databreaches.net Tuesday, is said to have happened in June when an unnamed hacking group claimed they used social engineering to trick an employee at a Marriott hotel Maryland into giving them access to their computer.

“Marriott International is aware of a threat actor who used social engineering to trick one associate at a single Marriott hotel into providing access to the associate’s computer,” Marriott spokesperson Melissa Froehlich Flood told TechCrunch in a statement. “The threat actor did not gain access to Marriott’s core network.”

Marriott said the hotel chain identified, and was investigating, the incident before the threat actor contacted the company in an extortion attempt, which Marriott said it did not pay.

The group claiming responsibility for the attack say the stolen data includes guests’ credit card information and confidential information about both guests and employees. Samples of the data provided to Databreaches.net purport to show reservation logs for airline crew members from January 2022 and names and other details of guests, as well as credit card information used to make bookings.

However, Marriott told TechCrunch that its investigation determined that the data accessed “primarily contained non-sensitive internal business files regarding the operation of the property.”

The company said that it is preparing to notify 300-400 individuals regarding the incident, and has already notified relevant law enforcement agencies.

This isn’t the first time Marriott has suffered a significant data breach. Hackers breached the hotel chain in 2014 to access almost 340 million guest records worldwide – an incident that went undetected until September 2018 and led to a £14.4 million ($24M) fine from the U.K’s Information Commissioner’s Office. In January 2020, Marriott was hacked again in a separate incident that affected around 5.2 million guests.

TechCrunch asked Marriott what cybersecurity protections it has in place to prevent such incidents from happening, but the company declined to answer.