A passenger travelling on a Princess Cruise cruise ship is believed to have fallen overboard, sparking a major search.

Officials said the 72-year-old man, believed to be an American citizen, was nowhere to be seen when the Ruby Princess ship arrived in San Francisco at 6.50am on Monday, at the end of a five-day voyage to Mexico that began on November 27. The passenger, who hasn't been identified publicly, was travelling alone and is feared dead.

Princess Cruises said it "ruled out other possibilities" and is treating the passenger's disappearance as a man overboard incident after company officials extensively reviewed CCTV footage, and the ship was searched several times without success. It is believed he disappeared in the early morning hours before the ship docked.

The man's disappearance was ruled as a man overboard incident, Princess Cruises said (

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According to media reports, US Customs and Border Protection took over the investigation once the ship docked in California. San Francisco TV station KRON4 reported that the United States Coast Guard is sending a plane into the air to search for the person 45 miles off the coast of Monterey, California.

Hunter Schnabel of the Coast Guard said: "What information we have to go off kind of does make it a little more difficult. To search a wide area of the ocean does take a lot of time and there is a lot that goes into that."

Princess Cruises said in a statement: "We extend our heartfelt condolences to the family and friends of the guest who was traveling alone." The remaining passengers on the ship disembarked on Monday. That evening, the ship departed on a 16-day voyage to Hawaii. The Ruby Princess is 195 feet high, has 19 decks and can hold more than 4,000 people on board.

In August, a young British man fell overboard on a cruise ship in the Adriatic Sea, sparking a desperate search. Croatian authorities said a passenger from the Royal Caribbean's Explorer of the Seas ship went missing in the early hours on Friday, August 9.

Days later, the search was suspended as authorities said they were "losing hope" after the incident. The President of the Croatian Sea Captains association said: "Since the man has been in the sea for over 40 hours, we are already losing hope. We would all like him to be found, but 40 hours is a very long time and we are losing hope that this search will end successfully."