(CNN) -- A tour bus carrying about 40 people rolled over Tuesday near Campbellton, Texas, killing two people and injuring more than a dozen, an official said.
Tommy Cavazos, a spokesman for the Texas Department of Public Safety, said that 18 people were injured in the Americanos tour bus crash on Interstate 37, about 50 miles south of San Antonio in southern Texas.
Most of the injured are being transported to San Antonio's Brooke Army Medical Center and University Hospital, Cavazos said.
The bus, which was headed to Matamoros, Mexico, flipped and landed on its right side, he said.
A spokesman for Brooke Army Medical Center, Dewey Mitchell, said four patients were taken to the hospital's trauma unit. He did not provide further information.
University Health System spokeswoman Julie Wiley said five adults -- four men and one woman -- were transported to the medical facility.
She could not describe their conditions but characterized the people as all between the age of 40 and 60.
Although it was not immediately clear whether others were taken elsewhere, Wiley said that University Health System and Brooke Army Medical Center are the two trauma centers in San Antonio and would receive patients in situations such as Tuesday's crash.
The names of those killed were not released.
Americanos is a subsidiary of Greyhound.
Maureen Richmond, a spokeswoman for Greyhound, said that 40 people were on board the bus, which left San Antonio shortly before 9 a.m. for Matamoros, Mexico. It was a regularly scheduled trip, she said.
She said the cause of the crash was not immediately known.
Video from the scene showed the bus lying on its side in the highway's grassy median, with clothing and other items scattered on the ground nearby.
Emergency personnel, including fire trucks and ambulances, were on the scene.
Mark Baldon, who came upon the crash shortly after it happened Tuesday morning, said he saw the bus "smoldering," CNN affiliate KSAT reported.
CNN's Divina Mims and Jamie Morrison contributed to this report.