It wasn't immediately clear what led to the incident. The U.S. Coast Guard was called in to help find the passenger, but has since called off the search.

The U.S. Coast Guard says it has suspended the search for a man believed to have gone overboard from a cruise ship as it was returning to San Francisco after a voyage to Ensenada, Mexico.

A 72-year-old passenger is believed to have gone overboard from The Ruby Princess cruise ship ahead of its arrival in San Francisco on Monday, Dec. 2

A 72-year-old man is believed to have gone overboard during a five-day Princess Cruise trip to Mexico, company officials said. The passenger's disappearance was ruled as a man overboard situation after company officials said they extensively reviewed CCTV footage and the ship was searched several times without success.

A 72-year-old man likely went overboard from a Princess Cruises ship before it docked in San Francisco this morning.  The Ruby Princess arrived in port at 6:50 a.m., Princess said in a statement to SFGATE.

The Coast Guard began aerial searches Monday afternoon in the Pacific Ocean about 45 miles (72 kilometers) off the coast of Monterey, California.

Image via Getty Images A cruise ship recently returned from a five-day voyage to Mexico. The only problem was that it was one passenger short, and no one knows what happened to him. A 72-year-old American man is missing.