President Donald Trump has used the death of a Border Patrol agent last November to call for his wall across the U.S. southern border, but the FBI announced on Wednesday there is no evidence the man's death was a homicide. 

Border Patrol Agent Rogelio Martinez, 36, was found dying in a culvert, lying next to his wounded partner, about 12 miles east of Van Horn, Texas, on November 18, the FBI said in a statement.

It remains unclear what transpired before the agents ended up in the rocky roadside water channel. But the bureau said Wednesday that after more 650 interviews—including those with all the federal, state and local emergency responders, and the dozens of medical personnel who cared for Martinez and his partner—"none of the...evidence collected and analyzed have produced evidence that would support the existence of a scuffle, altercation, or attack."

The manner of death remains undetermined, the FBI said.

Martinez's partner, Stephen Garland, who suffered serious head injuries during the incident, told investigators he is unable to recall specific details about what happened that night, according to The Washington Post

Garland was unclear even about his location when he radioed a dispatcher for help soon after he and Martinez were injured, the FBI said.

He reported: “We ran into a culvert,” “I ran into a culvert,” or “I think I ran into a culvert,” the dispatcher noted. The dispatcher also recorded in a Border Patrol log, “[He] thinks they [both agents] ran into a culvert.” 

Reviewing the autopsy at the request of the Post, San Francisco forensic pathologist Judy Melinek noted that Martinez appeared to have injuries only along the right side of his body, and no defensive wounds.

“The absence of defensive injuries and a single plane of injury is more consistent with an accident than a homicide,” Melinek said.

A day after the incident, Trump tweeted about the tragedy and used it to renew calls for a wall along the U.S.-Mexico border. 

"Border Patrol Officer killed at Southern Border, another badly hurt," he tweeted. "We will seek out and bring to justice those responsible. We will, and must, build the Wall!" 

Texas Senator Ted Cruz also said Martinez had been killed and used his death to call for increased security. 

"Our condolences and prayers go out to the family and friends of Border Patrol Agent Rogelio Martinez, who was killed this morning in the line of duty," he tweeted. "This is a stark reminder of the ongoing threat that an unsecure border poses to the safety of our community and those charged with defending them." 

The National Border Patrol Council, the union that represents border agents, issued a press release soon after the incident that appeared to discredit the notion that Martinez and Garland were injured by accident, as Melinek suggested. 

"Now, some news outlets...are intentionally attempting to bias the reader by writing the article with a preference toward the accident theory," the statement read.

Chris Cabrera, a spokesman for the organization, told the Post he still suspects the agents were attacked.

“We believe it was an assault on our agents and a murder of one of our agents and an attempted murder of another,” he said. 

The FBI said the investigation is ongoing. There is a $50,000 reward for information leading to the resolution of the case.