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 could not be found when it returned Friday morning.
Miami-based 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.
FBI spokeswoman Laura Eimiller said the couple was last seen Tuesday night near their cabin.
She said federal agents were interviewing passengers and crew and processing evidence.
The Coast Guard has been searching for the missing couple since Friday morning, said Petty Officer 2nd Class Prentice Danner. The search is being conducted by plane, not boat, because the area is so large, he said.
The ship left Long Beach on Monday for a cruise to Santa Catalina Island and Ensenada, Mexico.
Carnival spokeswoman Jennifer de la Cruz said she did not know if the couple disembarked in Ensenada or Catalina.
The couple's names and hometowns have not been released.
