VAN HORN, Texas (AP) — The FBI is investigating after one U.S. Customs and Border Protection agent died and another was seriously injured while on duty in South Texas.

Border Patrol spokesman Douglas Mosier says Agent Rogelio Martinez and his partner were taken to a hospital Sunday after they were hurt while responding to activity in the Big Bend area. The 36-year-old Martinez died at the hospital.

The agency has not released details about how the two agents were injured.

Border Patrol records show Big Bend accounted for about 1 percent of the more than 61,000 apprehensions its agents made along the Southwest border between October 2016 and May 2017. The region's mountains and the Rio Grande make it a difficult area for people to cross illegally into the U.S. from Mexico.