At least 12 people, including U.S. citizens, were killed when a bus carrying cruise ship passengers flipped over on a highway during an excursion to Mayan ruins in eastern Mexico.

The passengers, who had departed from South Florida, were heading Tuesday to the ruins at Chacchoben, about 110 miles south of Tulum, when the vehicle crashed on the Mahahual-Cafetal Highway in the southern part of Quintana Roo, according to Mexican authorities.

Quintana Roo state government officials said eight Americans, two Swedes and one Canadian were among the dead, according to the Associated Press.

Carrie Vanrenterghem told CBS News that she and her daughters passed the site as they were returning from the ruins.

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

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

The State Department confirmed Wednesday that a number of U.S. citizens had been killed and that several others had been injured, although the agency did not specify how many Americans were dead.

“We express our heartfelt condolences to those affected by this tragedy,” State Department spokeswoman Heather Nauert said on Twitter. “We are in contact w/ local authorities, have staff on scene to assist and will continue to monitor situation.”

Officials at the U.S. embassy in Mexico said in a statement Wednesday that they are also in contact with authorities there and “will continue to monitor the situation. We have staff on the scene and at local hospitals to assist victims and loved ones. We are aware of several U.S. citizens injured in the accident. We confirm the death of multiple U.S. citizens in a bus accident in Quintana Roo, Mexico.”

The AP reported that the Swedish Foreign Ministry and Global Affairs Canada confirmed that their citizens were among the dead.

The cause of the incident is under investigation, Mexican authorities said.

The bus was carrying 31 passengers, according to a statement from government officials in Quintana Roo.

Among those killed were a Mexican tour guide, a 78-year-old woman from Miami and a girl younger than 18, according to reports.

Officials said that at least 18 people were injured and taken to hospitals in Bacalar, Chetumal and Tulum.

The majority of the passengers came from two ships belonging to Miami-based Royal Caribbean Cruises: the Celebrity Equinox and the Serenade of the Seas, which both conduct seven-night Caribbean cruises, the company said in a statement.

About 23 passengers on the bus came from the Celebrity Equinox, which left Saturday from PortMiami, while four came from the Serenade of the Seas, which departed Friday from Port Everglades in Fort Lauderdale, Fla.

“Our hearts go out to all those involved in the bus accident in Costa Maya,” the company said. “We are doing all we can to care for our guests, including assisting with medical care and transportation.”

In addition to the passengers, the bus was carrying a driver and a guide, Costa Maya Mahahual, the bus company involved, said in a statement to the Associated Press.

The Chacchoben ruins are a popular tourist attraction, where people can explore ancient Mayan settlements, the earliest of which date to 1,000 B.C., according to the area’s website. Discovered in 1942, the site comprises several temples in natural condition surrounded by jungle and wildlife.

The bus was part of a fleet traveling to Chacchoben, said Chris Brawley, a Texas passenger on the Serenade of the Seas who was on another bus to the ruins that came upon the accident minutes after it occurred, according to the AP. It was shortly after 9 a.m., and the bus had been on the road for about 35 to 40 minutes.

The crash happened on a narrow, two-lane road with no shoulder or guardrail, Brawley said, and it did not appear as though any other vehicle was involved. The sky was clear.

Brawley said that he didn’t see the crash but that the “bus clearly lost control somehow,” as was evident by swerve marks on the road, he told the AP.

His ship had been in Roatan, Honduras, on Monday and was scheduled to be in Cozumel on Wednesday, he said.

Another passenger, Michael Schuenemeyer of Cleveland — who was on another bus that passed the accident an hour later — told the AP that the bus that crashed was “in pretty sad shape.”

More than a dozen accidents involving cruise bus excursions have happened near Caribbean ports of call over the past decade, Miami maritime lawyer James Walker of Walker & O’Neill told the Miami Herald.

In November 2016, a British tourist was killed in a bus crash in Dominica. Earlier that year, in January, a passenger was killed in a bus accident in Jamaica. In 2012, several people were injured in a bus accident in St. Martin.

In 2006, 12 people died in a bus accident in Chile after visiting a national park. Their tour had been arranged privately and was not among those the cruise line offered.

This post has been updated.

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