An ambulance sits parked next to an overturned bus in Mahahual, Quintana Roo state, Mexico, Tuesday, Dec. 19, 2017. The bus carrying cruise ship passengers to the Mayan ruins at Chacchoben in eastern Mexico flipped over on a highway early Tuesday. (Novedades de Quintana Roo via AP)

An ambulance sits parked next to an overturned bus in Mahahual, Quintana Roo state, Mexico, Tuesday, Dec. 19, 2017. The bus carrying cruise ship passengers to the Mayan ruins at Chacchoben in eastern Mexico flipped over on a highway early Tuesday. (Novedades de Quintana Roo via AP)

CANCUN, Mexico (AP) — A bus carrying cruise ship passengers on an excursion to Mayan ruins in southeastern Mexico flipped over on a narrow highway Tuesday, killing 11 travelers and their guide and injuring about 20 others, officials said.

Seven Americans and two Swedes were among the injured, said Vicente Martin, spokesman for the Quintana Roo state Civil Defense agency. Authorities had not yet established the nationalities of the dead.

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The bus ended up on its side in vegetation along the two-lane road. Video taken after the crash showed some survivors lying on the pavement and others walking around. One body lay on the roadside covered by a white sheet or other object, as the crash scene was cordoned off with yellow police tape.

Miami-based Royal Caribbean Cruises said in a statement that passengers from two of its ships, the Celebrity Equinox and Serenade of the Seas, were on the bus. The company expressed its sympathies and said it was assisting with medical care and transportation.

The U.S. Embassy in Mexico City said it was aware of reports that several American citizens had been injured and U.S. officials were working with local authorities to assess the citizenship and identity of individuals involved in the accident. It said the U.S. Consulate in Merida was ready to provide assistance.

Martin said investigators were working to determine the cause of the crash, which occurred as the bus was on its way to the ruins at Chacchoben, about 110 miles (175 kilometers) south of Tulum.

The Quintana Roo state prosecutor’s office reported that the fatalities were 11 tourists, including one child, and a Mexican tour guide.

It said in a statement Tuesday night that seven injured tourists had returned to their boat while 13 remained hospitalized, six of them in Tulum and seven in the city of Chetumal, near the Belize border.

The dead were being transferred to the forensics service for identification and subsequent notification of consular authorities.

Chris Brawley, a passenger on the Serenade of Seas, was on a bus headed to the same Mayan ruins when they came on the scene minutes after the accident.

It was shortly after 9 a.m. and a convoy of buses from the cruise port at Mahahual was headed to Chacchoben, he said. They had been on the road for about 35 to 40 minutes before the accident.

Brawly said the sky was clear and the road was dry and he didn’t see any sign of another vehicle being involved in the crash, which happened on a narrow, two-lane road with no shoulder or guardrail.

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He said he did not see the crash, but “the bus clearly lost control somehow as there were swerve marks all over the road surface.”

Brawley, of Haslet, Texas, said his ship departed Fort Lauderdale, Florida, on Friday for a seven-day cruise. They were in Roatan, Honduras, on Monday and were scheduled to be in Cozumel on Wednesday.

Michael Schuenemeyer, a minister from Cleveland, was on another bus that passed the crash about an hour later.

Tow trucks were lifting up the bus when they passed by, he said. “It was in pretty sad shape,” Schuenemeyer said.

Costa Maya Mahahual, the bus company involved, said in a statement that in addition to the tourists, a guide and driver were aboard the bus.

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Associated Press writer Gabriel Alcocer reported this story in Cancun and AP writer Christopher Sherman reported from Mexico City. AP writers Peter Orsi in Mexico City and Jennifer Kay in Miami contributed to this report.

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This story has been corrected to show the passenger’s last name is Schuenemeyer.