Authorities are searching Texas' Big Bend area for potential suspects and witnesses after a U.S. Customs and Border Protection agent was fatally injured responding to activity there.

Border Patrol spokesmen said they could not provide any details Sunday on what caused the agent's injuries or what led to them. Spokesman Carlos Diaz said the FBI has taken over the investigation.

Another spokesman, Douglas Mosier, said Agent Rogelio Martinez, 36, and his partner were transported to a local hospital, where Martinez died. Martinez's partner is in serious condition. His name wasn't released. Martinez had been a border agent since August 2013 and was from El Paso.

President Donald Trump commented about the incident on Twitter Sunday night amid a series of unrelated tweets.

"We will seek out and bring to justice those responsible," Trump's tweet read. He also reiterated his call for a wall at the U.S. border with Mexico.

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.

The Border Patrol website lists 38 agents who have died since late 2003 — some attacked while working along the border and others killed in traffic accidents. It lists one other agent death in the line of duty this year.