CHICAGO (CBS) — At least 12 people died when a bus carrying cruise ship passengers to Mayan ruins in eastern Mexico flipped over on a highway early Tuesday.

While the latest victims have not been identified, CBS 2’s Audrina Bigos reports that at least seven of the dead are Americans.

The tourism ministry of Quintana Roo state government says there were 31 people on the bus, with as many as 20 people possibly injured.

The crash occurred as the bus was on its way to the ruins at Chacchoben.

Video images from the scene showed the bus on its side in vegetation off the two-lane highway, CBS News reports, with some survivors lying on the pavement while others walked around. The road was narrow with no shoulder or guardrail.

CBS News Radio reporter Scott Mayman happened to be on board one of the cruise ships.

“There are some very grim scenes at the guest services area on board the ship. People consoling each other, still trying to figure out who was involved, as some of the tour buses have yet to return to the ocean liner,” he said.

In a statement, Royal Caribbean said, “The news from Costa Maya about a bus accident involving our guests is heartbreaking. We are doing all we can to care for our guests, including assisting with medical care and transportation.”

The U.S. State Dept. says it is aware of the situation, and U.S. officials are traveling to the site in Mexico.

The bus company, Costa Maya Mahahual, told CBS the nationalities of the passengers are: American, Italian, Swedish and Brazilian.

The cause of the accident is under investigation.