Japan Airlines said Thursday that its system network used to check in luggage is under a cyberattack, with at least 24 domestic flights delayed for up to an hour and some international flights also affected.

The airline said the problems began occurring around 7:25 a.m., prompting JAL to halt ticket sales for domestic and international flights for the rest of the day. Tickets already bought can still be used.

Investigative sources said the airline told police that it may have been the victim of a distributed denial-of-service, or DDoS attack, in which networks are overwhelmed by data from multiple sources over a short period.