Agent Rogelio Martinez, 36, was found clinging to life and his partner severely injured at the bottom of a culvert in a remote part of Texas 110 miles southeast of El Paso on November 18. Martinez died in a hospital in El Paso a few hours later.
"I find it hard (to believe) that a fall could have caused all the damage that he had", she said.
"Although disoriented and unsure of his location, the second Border Patrol Agent advised that both he and Agent Martinez were hurt".
"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. The assertion has been echoed by Texas politicians such as Gov. Greg Abbott, Sen. Ted Cruz - described the incident as "an attack".
The Border Patrol union has repeatedly said the agents were "ambushed".
On Twitter, President Donald Trump said the injured agent had been "brutally beaten" and called again for the construction of a wall between the United States and Mexico. The injured agent was released from the hospital but did not remember the incident. No suspects have been linked to the incident.
In the memo, McAleenan also indicated that no evidence suggested other people other than the agents were at the scene the night of the incident.
An official with the NBPC told The Los Angeles Times that after Martinez was injured, another agent called for backup while pursuing suspects.
Chris Cabrera, a spokesman for the National Border Patrol Council, said that despite the F.B.I.'s report and the autopsy results, he still believed that Mr. Martinez was attacked on the night of November 18. He also theorized that the agent's could have been sideswiped by a passing vehicle.
Early on in the investigation, the FBI said it was investigating the death as a "possible assault" but refused to rule out other possibilities, such as a fall or a auto accident.
Martinez died the next day from his injuries.
But the Federal Bureau of Investigation said it has found no evidence of a homicide, despite mobilizing significant resources involving 37 field offices to investigate Martinez's death. According to the FBI, Garland told a dispatcher, "We ran into a culvert".
During the investigation, the F.B.I. said that it identified two "persons of interest", but that a forensic analysis could not link them to the episode.
First responders found Martinez first, according to the internal Customs and Border Protection memo. The culvert's six channels are created to direct water beneath the highway.
"The height of the culvert to the area above the opening is approximately nine feet", the memo said.
Art Del Cueto, national spokesman for the union, said Wednesday the FBI's comments were preliminary.
"Our view hasn't changed". "I honestly believe they will", he said.
"It says that there is no evidence pointing it to being a struggle", Mr. Cabrera said in an interview. "It's just dumb, the physics aren't there". He doesn't have injuries on both sides of his body.
Culberson County Sheriff Oscar Carrillo, who responded that night, said it didn't look like an attack to him.
The FBI continues to offer a reward of up to $50,000 for information on the case and provides this number: 915-832-5000 to call with information.