(CNN) More than two months after the death of a US Border Patrol agent on duty in Texas, an internal government memo and the FBI appear to cast doubt on theories that the agent and his partner were attacked or ambushed.

US Border Agent Rogelio "Roger" Martinez and his partner, Stephen "Michael" Garland, were found near a concrete-lined culvert in southwest Texas on November 18. Martinez died in a hospital in El Paso a few hours later. Garland also suffered injuries.

"There were no defensive wounds" on Martinez and his partner and "there was no third-party blood or DNA evidence from the scene or from the agents' clothing," according to an internal memo, from acting US Customs and Border Protection Commissioner Kevin McAleenan, which was obtained by CNN.

The memo also states that due to his injuries, Garland's ability to recall the events has been impaired.

"The absence of evidence is a key factor in this case -- not due to lack of effort or determination, but because evidence which would indicate the presence of other persons or the commission of a criminal act is not present," the memo said.

Texas politicians -- including Gov. Greg Abbott and Sen. Ted Cruz -- described the incident as "an attack." A spokesman for the National Border Patrol Council, the union that represented the officers, said it was an "ambush." And President Donald Trump said the injured agent had been "brutally beaten" as he called again for the construction of a wall between the United States and Mexico.

But FBI investigators so far have found no evidence of an attack or scuffle in Martinez's death, the FBI said Wednesday, further deepening the mystery. The investigation, to date, however "has not conclusively determined" how the two agents ended up at the bottom of the culvert -- a tunnel structure used for water drainage -- according to Emmerson Buie Jr., the special agent in charge of the FBI's El Paso division. No suspects have been linked to the incident.

The only footprints at the scene belonged to the agents and first responders, the memo said.

The internal memo and the FBI's update on the investigation -- the agency's most detailed public account of the nighttime incident -- add another layer of uncertainty to the circumstances surrounding Martinez's death.

On Tuesday, a medical examiner in Texas said Martinez died from blunt injuries to his head, but the manner of his death is "undetermined."

Martinez's injuries included fractures to his skull, right jaw, upper ribs and his right collarbone, according to an autopsy report released by the El Paso County Medical Examiner's Office.

Sheriff Oscar Carrillo of Culberson County, where the two officers were injured, was on scene that night. He told the Dallas Morning News that investigators were looking at the possibility that the agents fell into the culvert in a nighttime accident. He also theorized that the agent's could have been sideswiped by a passing vehicle.

"The evidence is not obvious as to what happened out there," Carrillo said then.

The FBI has been considering several possibilities, including an accident, an attack or an altercation between the two agents, a Department of Justice official with knowledge of the investigation said.

Two people previously identified as persons of interest are not connected to Martinez's death, or his partner's injuries, the FBI said Wednesday.

The FBI said it conducted more than 650 interviews, including interviews with federal, state and local first responders, as well as dozens of medical personnel who treated Martinez and Garland.

Garland, who was disoriented and unsure of his location on the day of the incident, told the Border Patrol dispatcher he and Martinez were hurt, the FBI said.

"The second Border Patrol agent also made a statement to the effect of, 'We ran into a culvert,' 'I ran into a culvert,' or 'I think I ran into a culvert,'" the FBI said.

The dispatcher also wrote in a log: "(He) thinks they (both agents) ran into a culvert," the FBI statement said. The Border Patrol dispatch told Garland to go to his vehicle and activate his emergency lights so that first responders could find them, the FBI said.

First responders found Martinez first, according to the internal Customs and Border Protection memo.

"When Agent Martinez was found by first responders, it was 11:26 p.m. on a moonless night at the base of a culvert just off Interstate Highway 10.

"The height of the culvert to the area above the opening is approximately nine feet," the memo said.

"The evidence also shows that the other agent fell approximately 22 feet away from where Agent Martinez fell, landing on his back and sustaining significant injuries to his back and skull," the memo said.

A Customs and Border Protection spokesperson declined to comment on the case.

Chris Cabrera, a spokesman for the National Border Patrol Council, said the union disagreed with the findings layed out in the internal memo and the FBI's account.

"Our view hasn't changed. Our view is he was attacked," he told CNN, referring to the FBI's investigation. "It seems to me that they don't have any leads."

"There's no way he fell and dropped off that culvert," Cabrera said. "It's just dumb, the physics aren't there."

He pointed to the lack of injuries on the lower part of Martinez's body.

"If you fall from that height, you're not just going to land above the waist, your legs have to hit at some point," Cabrera said.

Cabrera questioned why the agents would have defensive wounds if they were ambushed.

"If you got hit from behind and didn't have a chance to defend yourself, then how would you defend yourself?" Cabrera said.

The FBI and the state of Texas have offered a $70,000 reward for information in this case.