A U.S. Border Patrol agent who was killed while on patrol in southwest Texas may have been beaten to death by attackers wielding rocks, according to the president of the National Border Patrol Council.

Brandon Judd, president of the labor union, said Agent Rogelio Martinez died Sunday of blunt force trauma to the head.

“I have been told by several agents that it was a grisly scene, and that his injuries were very extensive,” Judd said Monday in a phone interview with The Washington Post. “We believe he was struck in the head with rocks, or multiple rocks.”

U.S. Border Patrol agent Rogelio Martinez U.S. Border Patrol photo

Federal authorities have not yet said how Martinez was killed.

Authorities have been searching for witnesses and potential suspects after Martinez was killed and a fellow agent was seriously injured. Officials with Customs and Border Protection and the Border Patrol said in a statement that the pair were “responding to activity” near Interstate 10 in Van Horn, Texas, when they were harmed. The other agent called for help, and other agents provided medical care and took them to a hospital.

Martinez, 36, died from his injuries, officials said. The El Paso native had been a border agent since August 2013.

The other agent, who was not identified by authorities, remains in the hospital in serious condition, officials said.

Jeannette Harper of the FBI’s El Paso office, which is leading an investigation into the incident, told the San Antonio Express-News that authorities are still gathering evidence. She said a full account of what happened would not be released until Monday, but said reports that the agents were shot were not true.

“They were not fired upon,” she told the Express-News.

A Customs and Border Protection spokesman declined to provide any further details about the incident.

Judd, with the National Border Patrol Council, said Martinez got out of his vehicle about 4 a.m. Sunday to investigate an area near a drainage culvert along Interstate 10. The area is about 50 miles north of the border, he said, along a well-known smuggling corridor often used by drug couriers carrying backpacks full of marijuana.

Martinez called on his radio to report signs of suspicious activity, then began tracking what agents refer to as “signs,” typically footprints or other evidence of a recent disturbance to the soil or vegetation.

Following standard procedure, another Border Patrol agent in a separate vehicle headed to the area to provide Martinez with backup, Judd said.

Martinez appeared to have proceeded into the area near the culvert, as the areas are often used by smugglers to hide as they wait for a vehicle to pick them up. Judd said Martinez would have been an easy target for the attackers, as he was likely walking with his flashlight out and his eyes following footprints. At some point, he was ambushed, Judd said, and did not have time to draw his weapon.

Judd said the second Border Patrol agent arrived soon after and was also struck in the back of the head and beaten severely, but managed to use his radio to send a distress call.

That agent was found a short distance away from Martinez, Judd said.

“There was no appearance that either agent went for their weapon,” he said, adding that the attackers did not attempt to take the agents’ guns.

Martinez never regained consciousness and died during a helicopter evacuation to El Paso or soon after, Judd said. The second agent drifted in and out of consciousness.

“I have not heard he has regained memory of what happened,” Judd said. “So there is a possibility they were together when this happened, but we’re getting a lot of different information from different agents.”

By Monday afternoon, Texas Gov. Greg Abbott announced a $20,000 reward for information leading to an arrest and conviction for what he called the “murder” of a Border Patrol agent.

“We owe a great deal of gratitude to the brave men and women of the United States Border Patrol who serve every day to protect our homeland,” Abbott, a Republican, said in a statement. He added that as authorities conduct an investigation, “it is important that they receive any and all information to help apprehend and deliver swift justice to those responsible.”

Following news that an agent had been killed, President Donald Trump appeared to connect Martinez’s death to border security and plugged his plans for a border wall on Twitter on Sunday night.

“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!” Trump posted.

Sen. Ted Cruz, R-Texas, said on Twitter, without any further explanation, that Martinez and the other agent were “attacked” and he 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.”

Mexico said it would work with the United States during the investigation.

“The Government of Mexico expresses its deepest condolences to the family of Agent Martínez and hopes the injured agent makes a swift recovery,” Mexico’s Foreign Ministry said in a statement Monday, adding that the country “expresses its absolute willingness to collaborate with officials from the United States Department of Homeland Security if necessary.”

The FBI in El Paso is leading the investigation with help from the Culberson County Sheriff’s Department and Customs and Border Protection’s Office of Professional Responsibility.

The acting secretary of homeland security, Elaine Duke, has promised 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,” she said in the statement.

After the incident Sunday, a longtime friend of Martinez told ABC affiliate KVIA that Martinez was “the type of guy to give the shirt off his back and wouldn’t ask for anything in response.”

” . . . All he wanted to do was just help people and help the world and try to make a difference,” his friend, Emory Crawford, told the news station.

“He’s going to be missed by a lot of people,” he added. “I just wish him the best, that he rest in peace. I love him.”

The area where the two U.S. Border Patrol agents were harmed 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 1 percent of the roughly 61,000 apprehensions Border Patrol agents made along Texas’s southwest border between fall 2016 and spring 2017, according to the Associated Press.

Thirty-eight Customs and Border Protection agents have died in the line of duty since 2003, according to the agency’s memorial page.

Isaac Morales, who was stabbed in a bar parking lot in El Paso, was the only agent besides Martinez to die in 2017. Three agents died in 2016; two of them in car accidents, one of a heart attack while on bike patrol.