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A tourist bus carrying cruise ship passengers in Mexico killed 12 people and hurt 18 others when it flipped Tuesday. Seven Americans are among the injured and at least one American died in the crash.

The crash happened in eastern Mexico, about 40 minutes from the Costa Maya port, where the cruise ships were docked, reports CBS News’ Manuel Bojorquez. Investigators are still working to determine the cause of the crash that killed a tour guide and 11 tourists – including a child and a 78-year-old grandmother from Miami.

The tour bus crashed and flipped over just after 9 a.m. more than halfway into an hour-long trip from the port to the Mayan ruins of Chacchoben. Bodies could be seen strewn along the edge of the narrow two-lane highway after the accident. 

Local reports suggest the bus was traveling on the highway when it apparently drifted off the pavement. As the driver tried to get back on the road, the bus turned up on its side.

“When we went by it, it was pretty horrible. The bus, the front windshield was entirely out. Like, some people had fallen through it,” said witness Carrie Vanrenterghem.

She was traveling with her daughters back from the ruins when she came across the crash site.

“They were just traveling like we were, for you know, the holidays,” Vanrenterghem said. “It’s just hard to imagine their lives are gone.”

The excursion included travelers from two Royal Caribbean cruises: the Celebrity Equinox and Serenade of the Seas. CBS Radio reporter Scott Mayman was on board the Equinox. 

“It is an absolute tragedy,” Mayman said. “There were very emotional scenes on board the ship this afternoon as everybody re-boarded the vessel wondering what was going on.”

Among the dead is a 78-year-old grandmother from Miami. She was traveling with four other family members. Two were injured and the other two are unaccounted for.

Royal Caribbean released a statement saying: “Our hearts go out to all those involved in the bus accident in Costa Maya. We are doing all we can to care for our guests, including assisting with medical care and transportation.”

Authorities have not confirmed the nationalities of all those who were killed in the crash. The injured are being treated in local hospitals, but since the ships have left port, it’s unclear how they’ll get home if and when they recover. 

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