US authorities say a Texas border patrol agent has died and his partner was seriously injured while on duty.
They were attacked with rocks about 30 miles (48km) from the Mexico border, according to a labour union for border agents.
The FBI, which is leading the investigation, confirmed the agents “were not fired upon” with guns.
US President Donald Trump said he would “seek justice” for the agents and reiterated the need for a border wall.
In remarks during a cabinet meeting at the White House, Mr Trump said the border wall is necessary because “it’s rough territory” along the border.
“We’re going to straighten it out,” he said.
Both officers were taken to hospital, where one later died.
Mr Trump said the second officer injured on Sunday was “brutally beaten and badly, badly hurt”, but “looks like he’ll make it”.
In a news release issued late on Sunday, US Customs and Border Protection officials identified the slain agent as Rogelio Martinez, 36, of El Paso, Texas.
The agency did not release further details about his death, or name the other officer, referring questions to the FBI.
Texas Senator Ted Cruz and the state governor said the agents were the victims of an “attack” along a dusty stretch of road near the town of Van Horn in the Big Bend district.
Jeanette Harper, a spokeswoman for the FBI’s El Paso office, told the San Antonio Express-News the agents were not attacked with guns.
“They were not fired upon,” Mrs Harper said.
“There are so many different agencies working together that we need to come together and develop a timeline,” she added.
A representative for the National Border Patrol Council (NBPC) labour union told KTSM-News: “We strongly believe rocks were what was used.”
Chris Cabrera, a spokesman for the union, told NBC News that border agents are often attacked with rocks.
“We’ve had agents, a good friend of mine actually almost lost his eye,” he said. “It happens quite a bit, unfortunately.”
He added that some of the projectiles hurled at officers are as big as a softball.
Art Del Cueto, the vice-president of the NBPC, said the agents “were out there responding to a sensor and one agent was murdered”.
The NBPC – which says it represents 75% of the 18,000-officer agency – has been a strong proponent of President Trump’s border wall.
“You hear the talk about individuals trying to… enter this country to get a better life but the reality is they will stop at nothing including killing a federal agent – including taking someone’s life which is what happened,” Mr Del Cueto added.
Border patrol records show that the Big Bend region, where the apparent attack took place, only accounts for 1% of apprehended migrants.
The region’s mountains, and the Rio Grande River, make it one of the more difficult areas to cross the border illegally, CBS News reports.
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