A missing elderly couple likely fell off a Carnival Cruise ship between Long Beach and Mexico, company officials said Friday. A 90-year-old man and his 79-year-old wife were aboard the Carnival Paradise when it departed Monday from Long Beach, but they were missing when the vessel returned Friday morning.

Carnival said the couple’s cabin door was double-locked from the inside with a “Do Not Disturb” sign on the handle. Their belongings remained inside, and the door leading from the cabin to the private balcony was unlocked. They were last seen Tuesday near their stateroom.

The couple’s names and hometowns have not been released. The FBI is investigating the disappearance and is in the process of interviewing passengers and crew.