Last Updated Nov 21, 2017 4:50 PM EST
The U.S. Border Patrol is investigating the mysterious circumstances surrounding the.
Agent Rogelio Martinez died Sunday from injuries sustained in the line of duty. He and the second agent, who has not been publicly identified, were found late Saturday in a culvert near Van Horn, Texas, not far from the Mexico border.
The surviving agent is in stable condition, but he apparently does not remember anything that happened, reports CBS News correspondent David Begnaud. According to an internal Border Patrol email, it's unclear if they were attacked or fell into the culvert.
Martinez and his partner's injuries were extensive – broken bones and major head trauma. The injured officer is in critical, but stable condition, said Victor Velasquez, the acting chief of the U.S. Border Patrol's Big Bend Sector.
Martinez, 36, had injuries on his chest, collarbone and ribs. He had worked for the Border Patrol since 2013 and leaves behind his son, fiance and three stepchildren, Velasquez said.
"It's always hard when you lose a young and dedicated agent," Velasquez said.
Emmerson Buie, Jr., the special agent in charge of the FBI's El Paso division, said Tuesday the FBI is taking the lead on the investigation and is pursuing it as a "potential assault on a federal officer," though he stressed investigators don't yet have a clear picture of what happened.
He didn't rule out the possibility that the death was accidental.
"We're trying to gather the facts," Buie said. "If the facts support [an assault,] we will pursue it legally. If they support the incident was caused by something else, we will present that."
The Border Patrol agents' union believes the agents could have been victims of a stoning.
"It is consistent with having been assaulted with possibly rocks," said Art Del Cueto, National Border Patrol Council vice president Southwest. "That's the only thing they can think of. There is nothing else there on the scene."
The FBI said the agents were found around 11:20 p.m. Saturday near Interstate 10. Earlier that night, a law enforcement source tells CBS News that Martinez radioed in that he was getting out of his vehicle and following a trail of possible footprints in the dirt.
Martinez was found unconscious and bleeding from the head. He apparently had not drawn his weapon.
"He was a dedicated agent that went out there to do his job and he did not make it home," Del Cueto said.
Buie said the FBI is considering the area a crime scene.
The agents were airlifted to a hospital in El Paso where Martinez died from his injuries.
"All he wanted to do was just help people and help the world and try to make a difference," said Emory Crawford, Martinez's longtime friend. "He would be the type of guy to give his shirt off his back and wouldn't ask for anything in response."
On Monday, President Trump offered his sympathies to the agents' families and renewed his call for a border wall.
"We have to stop the massive drug flow from pouring in, and my respect to the families that were so badly hurt because they were devastated," Mr. Trump said.
The FBI is asking the public for any and all tips. Texas Gov. Greg Abbott is offering a $20,000 reward for information leading to an arrest. Tuesday, Buie announced an additional reward of $25,000 for information that leads to "an arrest and conviction" in Martinez's death.