A U.S. official with knowledge of the investigation into the death of a border patrol agent in South Texas says the surviving agent who radioed for help doesn't remember what happened.

The official, who was briefed on the investigation but is not authorized to speak publicly and spoke on the condition of anonymity, said Monday that investigators believe agent Rogelio Martinez may have fallen into a 14-foot culvert. Martinez died early Sunday.

The FBI says autopsy results are pending. Martinez's partner remains hospitalized.

The official says the incident occurred after dark in an area that's known for drug activity and where agents often look for drugs in culverts.

In a statement Monday, FBI spokeswoman Jeanette Harper says both agents had traumatic head injuries after being found in a culvert near Van Horn, Texas.