The investigation into the death of a US Border Patrol agent has yielded no evidence that there was a “scuffle, altercation or attack”, FBI officials said.
It comes more than two months after US president Donald Trump used the suggestion of an attack to promote the building of a wall along the US-Mexico border.
Investigators have conducted more than 650 interviews and involved 37 field offices in their probe, but have not found definitive evidence of an attack, the FBI said in a statement.
The investigation will continue and the reward of up to 50,000 US dollars (£36,000) for information that closes the case will remain.
“To date none of the more than 650 interviews completed, locations searched, or evidence collected and analysed have produced evidence that would support the existence of a scuffle, altercation, or attack on November 18, 2017,” said the release from the El Paso office of Emmerson Buie, a special agent in charge.
Rogelio Martinez died from injuries he sustained while he and his partner were responding to reports of unknown activity on November 18 near Van Horn, a Texas town near the Mexico border about 110 miles south-east of El Paso.
Mr Martinez’s partner radioed for help before both agents were airlifted to hospital, where 36-year-old Mr Martinez died a few hours later. The partner, who suffered head injuries, was released after several days but told investigators he could not remember the incident.
A post-mortem report from the El Paso County medical examiner’s office released on Tuesday night shed little light on what caused the serious injuries that Mr Martinez suffered. The report listed “blunt force trauma” as the cause of death, but said the manner of his death was undetermined.
Mr Martinez was found to have sustained a skull fracture, a fractured eye socket, multiple rib fractures and a broken collarbone.
A border patrol union, the National Border Patrol Council, has previously said the pair were attacked and struck with a rock or rocks.
Mr Trump, Senator Ted Cruz and Texas Governor Greg Abbott have said the two were attacked, with Mr Trump using the incident to renew his call for a border wall between the US and Mexico.
Mr Trump has repeatedly linked immigration to criminality in making his case for tighter border security.
Investigators had looked into tips about several people of interest, including two in New Mexico. While other charges were filed against several individuals, none of those charges are related to Mr Martinez’s death, the FBI said, adding that they are not “currently a subject in the investigation”.