A 72-year-old American man disappeared while traveling on a Princess Cruise ship, and officials suspect he fell overboard during his time on the ship.

A search began on Monday at about 7 a.m. after the Ruby Princess Cruise ship docked in San Francisco and crews realized the man could not be found onboard, according to the cruise line.

The cruise ship left for Ensenada, Mexico, on Nov. 27.

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The cruise ship left for Ensenada, Mexico, on Nov. 27. (Getty Images)

The man was traveling alone on the cruise. It remains unclear when he may have gone overboard.

"Princess worked throughout the day with local authorities. CCTV footage has also been extensively reviewed and the ship was searched thoroughly several times without success," a spokesperson for the cruise line told the New York Post.

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The missing man was traveling alone on the cruise. (Getty Images)

"Having ruled out other possibilities, this is being treated as a man overboard incident," the spokesperson added.

U.S. Customs and Border Protection is investigating the incident, a spokesperson for the cruise line told KTVU. The spokesperson said Princess Cruises is cooperating with authorities in the investigation.

U.S. Customs and Border Protection is investigating the incident. (Getty Images)

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The U.S. Coast Guard told the outlet that as of Monday at about 5:30 p.m., they had suspended their search for the missing man.