DALLAS — A U.S. official with knowledge of the investigation into the death of a border patrol agent said Monday that the surviving agent who radioed in 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 Rogelio Martinez was found at the bottom of a 14-foot culvert and that investigators believe he may have fallen. The official said the incident happened after dark in an area is known for drug activity and where agents often look for drugs in culverts.

A spokeswoman for the FBI, which is leading the investigation, said Monday in a statement that both agents were found late Saturday night in a culvert near Van Horn, Texas, and that both had traumatic head injuries.

FBI spokeswoman Jeanette Harper said Martinez died early Sunday. Results of his autopsy are pending.

Authorities haven't offered an official explanation of what happened over the weekend in West Texas, and a border patrol supervisor said reports the agents were attacked are "speculation."

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Several elected officials, including President Donald Trump, have called Martinez's death an attack. Rush Carter, a supervisor for the U.S. Customs and Border Patrol region that includes the area where Martinez died, said all the agency can confirm is that the two "were injured while performing their regular duties."

"We are waiting for the investigation to fully determine how those injuries happened," Carter said Monday night.

CBP issued a statement Sunday that was thin on details, saying the two agents "were responding to activity" near Interstate 10 and close to Van Horn, about 30 miles from the border with Mexico and 110 miles southeast of El Paso. A CBP spokesman said Martinez and his partner were taken to a hospital, where Martinez died. Martinez's partner, whose name hasn't been released, was in serious condition.

Kevin McAleenan, acting commissioner of CBP, said in a letter sent to border agents on Sunday that Martinez was unconscious when agents found him with "multiple injuries" to his head and body.

Harper said on Sunday that Martinez and his partner were "not fired upon," but she didn't elaborate.

However, Chris Cabrera, a spokesman for a border patrol agents union, the National Border Patrol Council, told The Associated Press that the two agents appeared to have been struck in the head with a rock or rocks. Cabrera said agents who responded to the scene described it as "grisly" and said Martinez and his partner had "extensive injuries."