An F.B.I. investigation into the mysterious death of a Border Patrol agent found unconscious off a remote West Texas highway in November has been unable to determine how he was injured, though the bureau said on Wednesday that it had found no evidence suggesting he had been attacked.

In a statement, the Federal Bureau of Investigation said it could not find anyone who had witnessed the episode in which the agent, Rogelio Martinez, was killed besides his partner, Stephen Garland, who survived but has no recollection of what happened.

“This investigation has not conclusively determined how Mr. Martinez and his partner ended up at the bottom of the culvert and no suspects have been linked to this incident,” the F.B.I. said.

Just hours after the agents, who had been on duty, were found in a dark culvert off Interstate 10, about 100 miles east of El Paso, politicians seized on the news as a flash point in a broader debate about immigration and border security. President Trump called it an “attack” and further evidence that a wall was needed along the United States’ border with Mexico. Senator Ted Cruz of Texas said it underscored the constant dangers of safeguarding often rugged and remote terrain.