Martin said the crash occurred as the bus was on its way to the ruins at Chacchoben, about 110 miles (175 kilometers) south of Tulum.
The cause of the accident was under investigation.
Costa Maya Mahahual, the bus company involved, said in a statement that the bus was carrying tourists who had arrived to the coastal town of Mahahual aboard a cruise ship.
A spokesman from the U.S. Embassy in Mexico said U.S. officials were working with local authorities to determine whether U.S. citizens were involved.
Video images from the scene showed the bus on its side in vegetation off the two-lane highway, with some survivors lying on the pavement while others walked around. Sky More