Authorities were searching southwest Texas for suspects or witnesses after a U.S. Border Patrol agent was killed and his partner injured Sunday while on patrol in the state's Big Bend area, officials said.
Martinez's partner called for help. Other agents arrived, provided medical care and took them to a hospital.
Martinez died of his injuries; his partner, who was not identified, remained in the hospital in serious condition, officials said.
Martinez, a 36-year-old from El Paso, had been a border agent since August 2013.
Jeannette Harper of the FBI's El Paso field office told the San Antonio Express-News that authorities were still gathering evidence. She said reports that the agents were shot were not true, but that a full account of what happened wouldn't be released until Monday.
"They were not fired upon," Harper said.
A Customs and Border Protection spokesperson declined to offer any further details about what happened.
Art Del Cueto, vice president of the National Border Patrol Council labor union, told NBC affiliate KTSM that the two agents were responding to an electronic sensor when they were attacked. Although officials have not yet released details, Del Cueto said, "we strongly believe rocks were what was used."
"I know people don't want to jump to conclusions and say stuff . . . but you hear the talk about individuals trying to get a better life and 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," Del Cueto told the station.
President Donald Trump appeared to connect Martinez's death to border security, and plugged his plans for a border wall Sunday night on Twitter.
Trump tweeted "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!"
Sen. Ted Cruz, R-Texas, asserted, without explanation, that Martinez and his partner were "attacked" and also linked the incident to security on the border with Mexico.
"This is a stark reminder of the ongoing threat that an unsecure border poses to the safety of our communities and those charged with defending them," Cruz tweeted. "I remain fully committed to working with the Border Patrol to provide them with all the resources they need to safeguard our nation."
The FBI in El Paso is leading an investigation into the incident, along with the Culberson County Sheriff's Department and Customs and Border Protection's Office of Professional Responsibility.
Acting Secretary of Homeland Security Elaine Duke said in a statement that she learned of Martinez's death Sunday morning, and offered her agency's full support to "determine the cause of this tragic event."
"On behalf of the quarter of a million front line 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," Duke said.
The area where the agents were injured is a dusty stretch of highway about 100 miles east of El Paso.
It is part of Customs and Border Protection's vast Big Bend Sector, which covers 135,000 square miles in Texas and Oklahoma and 510 miles of river border. The sector's Van Horn Station, near where Martinez died, covers 15 miles of the Mexico border.
The Big Bend Sector accounted for one percent of the roughly 61,000 apprehensions Border Patrol agents made along Texas's southwest border between fall 2016 and spring 2017, as the Associated Press reported.
Local media photos from the scene showed Border Patrol trucks and about a dozen other unmarked vehicles parked along the side of the road, and a group of law enforcement agents huddled together.
Thirty-eight Customs and Border Protection agents have died in the line of duty since 2003, according to the agency's memorial page.
Before Martinez, the only other agent to die in 2017 was Isaac Morales, who was stabbed in a bar parking lot in El Paso. Three agents died in 2016, two of them in car accidents, one of a heart attack while on bike patrol.