A bus crash in Mexico left 12 people dead and at least a half dozen injured, according to authorities. The bus was carrying tourists from two Royal Caribbean cruises, which left South Florida last week, to see Mayan ruins.

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The 2,850-passenger Celebrity Equinox sailed from Miami on Dec. 16 for seven nights in the western Caribbean, while the 2,490-passenger Serenade of the Seas left Port Everglades in Fort Lauderdale on Dec. 15 for seven nights in Key West, Honduras and Cozumel, the Miami Herald reported.

The tour of the ruins in Chacchoben is sold to passengers through the cruise lines. The bus carrying 27 Royal Caribbean passengers flipped on a two-lane highway, officials said. A guide and driver were also aboard the bus.

It was one of the deadliest incidents ever related to a cruise ship excursion, several experts told the Herald. Authorities have not released names or nationalities of those killed. Seven Americans and two citizens of Sweden are among those injured, the Herald reported.

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"Our hearts go out to all those involved in the bus accident in Costa Maya,” Miami-based Royal Caribbean said in an official statement. "We are doing all we can to care for our guests, including assisting with medical care and transportation."

The cause of the crash is under investigation. The road was reportedly dry and no other vehicles were involved.