(CNN) - Details remain scant in what the FBI describes as a "tragic incident" that left one Border Patrol agent dead and another in serious condition.
The FBI said both men were found injured late Saturday in a culvert area in southwest Texas.
Rogelio Martinez, 36, died from his injuries at a hospital Sunday morning. His partner, who hasn't been identified, remains hospitalized in serious condition.
The two Border Patrol agents suffered traumatic head injuries and other wounds such as broken bones, according to the FBI, which is leading the investigation. The results of Martinez's autopsy are pending.
Although there hasn't been an explanation of what happened, Texas politicians -- including Gov. Greg Abbott and Sen. Ted Cruz -- have described the incident as an "attack." President Donald Trump said the injured agent had been "brutally beaten" as he called for the wall between US and Mexico.
But in Culberson County, where the two officers were injured, the local sheriff painted a different picture, suggesting to the Dallas Morning News that investigators are considering the possibility that the agents fell in a nighttime accident. "The evidence is not obvious as to what happened out there," Sheriff Oscar Carrillo told the paper.
Martinez and his partner had been on patrol near Interstate 10 in the Big Bend Sector, which runs along the US-Mexico border, when they responded to activity, US Customs and Border Protection said. CBP officials would not say what type of activity.
After they responded, Martinez's partner reported that they were both injured and needed help, according to CBP. They were found about 12 miles east of the town of Van Horn at 11:20 p.m. Saturday, according to the FBI.
A Customs and Border Protection spokesman said Martinez had not been shot.
He had been a border patrol agent since 2013, officials said.
President Donald Trump cited the incident as a reason why the US needs to build a wall along the border with Mexico.
"As you heard we lost a Border Patrol officer just yesterday and another one was brutally beaten and badly, badly hurt," Trump said Monday. "It looks like he'll make it, but very badly hurt."
On Sunday, Trumped had tweeted, "We will seek out and bring to justice those responsible. We will, and must, build the Wall."
On Monday, Gov. Greg Abbott's office announced a reward of up to $20,000 through Texas Crime Stoppers for information leading to the arrest and conviction of whoever killed Martinez and wounded his partner.
He tweeted: "I'm offering a reward to help solve this murder of a Border Patrol Agent in Texas. Help us catch this killer."
Texas Sen. Ted Cruz said the attack was "a stark reminder of the ongoing threat that an unsecure border poses to the safety of our communities and those charged with defending them."
Mexico expressed its regret regarding the incident, its willingness to collaborate with the US Department of Homeland Security and condolences to the families of the two agents.
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