The driver, who was from Guadalupe, Nuevo Leon, Mexico, tried to bring an elephant skull into Mexico, which didn't allow him to cross with the skeletal remains, according to CBP.
Back on the U.S. side of the river, the driver, who isn't named in a press release issued Monday, couldn't provide exportation documentation or proof of ownership. The driver also lacked proper U.S. Fish and Wildlife Service permits for the skull.
Federal officials made the find at an international bridge in Pharr, Texas, in the Rio Grande Valley.
The skull belonged to an African bush elephant, a popular target for poachers seeking ivory in Africa. The species' population is listed as vulnerable, but their numbers are increasing, according to the World Wide Fund for Nature, a conservation organization.
There are approximately 470,000 African elephants living in the wild.
Agricultural specialists with U.S. Customs and Border Protection made a colossal find after Mexican authorities sent a truck driver attempting to enter that country back to the northern side of the Rio Grande.