A U.S. Border Patrol agent died Sunday from injuries sustained while patrolling in Texas’ Big Bend sector near the U.S.-Mexico border.

Agent Rogelio Martinez and his partner were responding to “activity” while on patrol near Interstate 10 when both reported they had been injured and were in need of assistance, according to a statement from Customs and Border Protection (CBP).

Responding agents transported both wounded to a local hospital, where Martinez passed away from his injuries. His partner remains hospitalized in serious condition, CBP said.

Officials have not released further details about what caused Martinez’s injuries or what kind of activity the agents were responding to when they were hurt. The Border Patrol’s Special Operations Group and CBP’s aviation and marine assets are searching the area for potential suspects or witnesses.

Shortly after CBP confirmed Martinez’s death, President Donald Trump said the incident showed the need for a wall along the Southwest border.

“Border Patrol Officer killed at Southern Border, another badly hurt. We will seek out and bring to justice those responsible. We will, and must, build the Wall!” the president tweeted Sunday night.

Martinez, 36, of El Paso, Texas, had been a border agent since August 2013.

Acting Homeland Security Secretary Elaine Duke called Martinez’s death a “tragic event” and said the the administration is “fully supporting the ongoing investigation” of the incident.

“On behalf of the quarter of a million frontline officers and agents of DHS, my thoughts and prayers go out to the family and friends of Agent Martinez and to the agent who is in serious condition,” she said in a statement.

The Big Bend sector is not known as one of the Border Patrol’s most active, accounting about 1 percent of the more than 61,000 apprehensions the agency made along the border from October 2016 to May.