The technical problem at Sunwing Airlines Inc. that has grounded flights and stranded thousands of passengers this week is being blamed on a data security breach of the carrier’s third-party service provider, Airline Choice.

Sunwing said on Wednesday that it was continuing to check in passengers manually after the cyber attack, which caused disruptions that left many travellers languishing in airports at the start and end of vacations largely to sun destinations in the Caribbean, Mexico, and Central America.

Sunwing said it is working with other airlines to source additional aircraft to help relieve the backlog in some airports, tweeting, “We’ve… sub-chartered aircraft from other carriers, including WestJet, Air Transat and Nolinor Aviation, to help support our flight operations.”

Airline Choice is an Illinois-based company that provides air carriers, including Virgin Atlantic, Aruba Air, and Omni Air, with a technological platform aimed at streamlining the passenger handling, self-service, and compliance process. Sunwing is the company’s only Canadian client.

In an emailed statement Wednesday, Airline Choice described the breach as a “data security event” that affected a limited number of its computer systems.

“As a precaution, we took certain systems offline to secure our environment. We also immediately launched an investigation to determine the nature and scope of the event,” the company said.

Airline Choice added it has already begun restoring functionality to its systems and is working around the clock until that is complete.